The isolation of Human worlds from each other was due to the growth of the Chaos Power Slaanesh which accompanied the Fall of the Eldar. Slaanesh's growth caused colossal disturbances in the very fabric of the warp, better known as warp storms. These warp storms made warp travel unpredictable and dangerous and eventually impossible. Without the ability to warp travel, travelling any distance further than nearby planets and very close solar systems became unfeasible due to the time required to complete the voyages using other conventional engines. So were the planets of mankind isolated from one another for nearly 5000 years.

The techno-barbarian warriors that dominated Earth wore this type of armour. When the Emperor began his conquest of Earth his retinue was equipped and armed in the same way as other warlords. The first Space Marines formed part of that retinue and were equipped with the same sort of armour as other warriors of the time.

The Thunder-bolt and lightening emblem on the breastplate of this suit was the personal badge of the Emperor in those days, predating the Imperial eagle which only became the symbol of the Imperium much later. This emblem gives the armour its common name - Thunder Armour.

Thunder Armour was not a single enclosing suit and offered no atmospheric protection or life-support facilities - all of these being unnecessary while fighting was restricted to Earth. The armour was typically adorned with a helmet with a top plume, but all these early suits were manufactured on an entirely local basis and there exact designs were often a matter of personal taste. The main part of the armour is the massive powered torso which encloses the chest and the arms. Beneath the armoured chest plate coiled energy cables transmitted power into the arms, effectively increasing the wearer's fighting abilities by three or four times over. During this period most fighting consisted of close combat, warriors preferring to grapple with each other rather than use long ranged weapons - the power of a warrior's chest and arms being of paramount importance.

The warrior's legs are not power armoured at all but enclosed in tough padded breeches. Armoured greaves and armoured boots were standard for the early Space Marines although not for many warriors of the time. Warriors with Thunder Armour wore a backpack which provides the suit with power - most of its bulk taken up by a cooling mechanism meant to prevent the power unit from overheating.

Once the Terran system was secure and the process of rebuilding firmly in hand, the galactic conquest could begin. Even before the warp storms and the Age of Strife ended, the Emperor started to make provisions for his Great Crusade. Part of these plans included the re-equipping of the Space Marine armies with a far more sophisticated fighting suit.

With its advanced technology the newly conquered planet of Mars became the centre for munitions development. New types of armour were produced in great numbers in the Martian factories under the direction of the Adeptus Mechanicus, the ruling class of Tech-priests installed to administrate its affairs on behalf of the Emperor. This enabled the entire Space Marine Corps to be re-equipped.

The new type of armour was the Crusade Suit, which soon became known as Mark 2 armour. The armour is totally enclosed and life sustaining, and so suitable for fighting on alien worlds as well as in deep space. It is arranged into articulated hoop-shaped plates for ease of movement and these now cover the legs as well as the chest. The additional energy cabling required to operate the leg armour is exposed, while the chest armoured plates enclose coils. The old armour had deliberately positioned the power cables on the outside of the armour to help keep the armour cool, but more efficient coolers in the Mark 2 made this unnecessary.

The backpack retained the old shape but is much more efficient and contains all the extra equipment needed to maintain life-support, air recycling, fluid recovery, and various automatic medical functions which have remained standard to Space Marine armour ever since.

The helmet was also fitted with automatic sensory devices developed in the Martian workshops. These consist of exterior sensors which gather visual and audio stimuli from the immediate environment - effectively functioning as eyes and ears. The information gathered in this way is processed by a computer brain and then transmitted directly into the wearer's mind by a neural connector. The practical result for the wearer is that he appears to see and hear quite normally, but he can also see infrared and ultra-violet light, and hear a wider range of sound frequencies. The wearer is also able to selectively enhance a visual image or sound should he wish.

If exposed to blinding lights or deafening noises, the computer processor acts as a safety valve and dampens down the stimuli preventing damage to the Space Marine.

The Mark 3 armour variant dates from the inner-galactic wars between the Emperor's forces and the inhabitants of the worlds close to the galactic core, which included many of the Squat Homeworlds, not all of which were pleased to find themselves the object of galactic reconquest. Mark 3 armour was never intended to replace Mark 2, but to provide an optional heavy armour type for suitable for fighting on board spacecraft and in tunnel complexes. High casualties suffered during early battles had shown the need for such armour. Mark 3 armour therefore placed considerable emphasis on frontal protection, while the rear armoured plates were lightened to compensate. This armour was reckoned ideal where cover was minimal and combat was a matter of frontal assault.

The suit itself is a highly modified Mark 2 with the addition of fixed armour plates to the body and limbs and a new heavy armoured helmet. The sloping plates of this helmet were intended to deflect shot to the left and right, and was to inspire the Mark 4 and 6 helmet designs. No Space Marine forces were equipped solely with this mark although many modern Chapters still use Mark 3 armour for boarding actions and tunnel fighting.

The Great Crusade lasted for approximately 200 years at the end of which came a period of political consolidation. The Space Marines were now scattered far and wide throughout the galaxy, many serving as garrisons rather than as campaigning armies and their size was scaled down to reflect this new role. Much of the equipment of the past was rapidly wearing out, including the old Mark 2 and 3 armour produced on Mars. While some Marine Chapters chose to continue local maintenance and production, the Martian factory hives of the Adeptus Mechanicus set about production of a new variant. This was to be the Mark 4 or Imperial Maximus Suit.

The main change was to abandon the separate abutting plates in favour of larger inflexible joints originally developed for the Mark 3. The result was only marginally less mobile than the earlier type and considerably easier to produce and maintain. Technical secrets uncovered on newly conquered worlds enabled the Martians to develop a more efficient armour, improving the quality of protection and reducing the weight of the suit at the same time. Improved armouring of the power cables enabled the main arm and chest supply to be safely relocated on the exterior of the armour, while use of new material also allowed the size of and number of cables to be reduced.

The general issue of Mark 4 armour was only half complete when the Horus Heresy broke out. This threw the entire program of supply into turmoil. In fact many of the most recently supplied Chapters were to turn against the Imperium while many loyal Chapters were forced to continue with older variants, and the confusion was considerable. The Space Marine armours, namely Techmarines and Artificers, had hardly got used to the new armour and many were yet unable to maintain it properly let alone duplicate it as was originally intended.

With the Mark 4 newly in service the need for large numbers of spares had not been anticipated, so that suits quickly became unusable due to quite minor battle damage. It was soon found that the new and rather specialised materials used in the construction of the Mark 4 were unavailable locally and this increasingly became a problem as Chapters moved from battle-zone to battle-zone. The Imperial forces were soon forced into a fallback position. Production of Mark 4 armour ceased and a new type of armour was designed literally overnight. This was the Mark 5 or Heresy Suit.

The Mark 5 used as many pre-Mark 4 components as possible. Large stocks of these existed and the Marine Artificers were already familiar with their application. Once supplies of the new materials used in the Mark 4 armour dried up it became necessary to re-use older substances. The lighter chest, arm, and leg cabling of the Mark 4 was replaced by older and heavier style cabling made from more readily available materials. However the cables were now exposed because they were too bulk to fit under the new style chest plate. This was to prove a consistent weak spot in the design leading to the fitting of all kinds of improvised chest armour.

A distinguishing feature of the Mark 5 armour were the heavy studded armour plates. This was an attempt to reinforce the Mark 4 pattern plates when inferior materials were used due to the lack of proper supplies. An extra skin plate was fitted around the armour using molecular bonding studs. The extra weight was considerable, especially if a further chest plate had been added, leading to increased pressure for energy from the power pack. As a result the wearer either had to turn up the power output and suffer intolerable heat build-up, or leave the power supply as it was and accept reduced power levels.

The helmet type is a spin off from the Terminator development program, an early type of production helmet, sharing the same type of auto-sense components as contemporary Terminator suits. Being something of an improved stopgap, it is common for Mark 5 suits to vary a great deal. Where Mark 4 helmets, armoured plates and cabling were available these were often used.

At the same time as production of Mark 4 armour ceased, work began on a long-term development program to replace the Mark 4 with a more durable type. The Mark 5, or Corvus Suit, was only ever perceived as a stopgap design. The weapon development workshops on Mars began to experiment with a mixture of new and old technology, making the newer materials more durable where possible.

A notable feature of the resulting armour types (Mark 6 and 7) is the provision of dual technology circuits. These permit relatively rare or sophisticated functions to be temporarily replaced or repaired using common or very simple technology. Although development was incomplete the new armour was rushed into production while the forces of Horus advanced throughout the Terran solar system. Hastily equipped Space Marines wore the new style Mark 6 armour into battle while the development laboratories where disassembled and prepared for transfer to Earth. During the Martian campaign forces of Horus eventually overran the production facilities for the Space Marine armour and soon began to manufacture new suits for their own use. Consignments were distributed to other forces elsewhere in the galaxy so that this new type of armour became quite widespread.

While the final battle for Mars was underway the Imperium, realising that the planet would eventually fall, set about duplicating the munitions production lines back on Earth. The armour development teams from Mars were transferred wholesale to continue the development program and incorporate their latest work into a new armour type. As Horus' forces finally overcame the defences of Mars new Mark 7 armoured suits started to reach the Space Marines on Earth and the Moon. Mark 7 represents the fulfilment of the new design program which was really only half complete in the Mark 6. In fact, so effective was the Mark 6 that both types continued in service thereafter and many Chapters chose to continue with their old armour rather than adopt Mark 7.

The 7 basic marks of Space Marine armour were all developed up to and during the period of the Horus Heresy. During the production history of each mark various improvements were incorporated in the light of field experience. Thus there is a certain variation even within each mark although this is usually limited to the types of materials used rather than to stylistic changes.

Following the end of the Heresy much of the Imperium changed, including the organisation and number of the Space Marine Chapters. Whereas up until this time there had only been twenty Chapters, henceforth the huge pre-Heresy forces were broken up into many smaller Chapters. The new Chapters that were founded were equipped with whatever suitable armour and weaponry was available. For the most part the armour used was either Mark 6 or 7, but with a fair sprinkling of older types.

Since that time each Chapter has largely taken over production of its own equipment. This is not to say that every Chapter produces every single item of hardware that it uses. Some Chapters trade items with other Space Marine Chapters, or they commission work from local fabricators. This later option is especially common where Chapters hold the governorship of the world they live on - in which case the planet is effectively owned by the Chapter and the resources can be organised by the Space Marines as they wish. In other Space Marine Chapters supplies are purchased through the Adeptus Mechanicus.