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The surgical process takes a great deal of time. The recruit receives implants, along with chemical- and hypnotherapy, and training necessary for allowing the functioning and development of the implanted organs. The implants transform their bodies and minds and give them inhuman abilities - making them capable of spitting acidic venom, absorbing the memories of the dead by eating their flesh, raising them to heights of up to three metres tall,{{Fn|50}}, darkening their skin to protect it from radiation, and operating for long periods without sleep by switching off parts of their brains at a time. After this implantation process and the associated training, the recruit becomes an "initiate".

Space Marines represent the pinnacle of Imperial bio-engineering. They are larger, stronger, faster,{{Fn|51b}}, and more durable than average humans. Most also display higher cognitive abilities, such as very accurate powers of recollection and a capacity to process information within mere nanoseconds.{{Fn|53}}{{Fn|62b}} Space Marines have frightening strength, which when augmented by power armour allows them to act as living battering rams capable of smashing through fast-moving light vehicles or crush human heads into pulp.{{Fn|55}} Space Marines also display superior vision, hearing, smell, sun/radiation resistance, and taste than standard humans. They have far greater resistances to heat and cold and can even survive for a time in the vacuum of space.{{Fn|2}}

Fully armoured, a Space Marine typically stands 2.1 meters (6'8") to 3.0 meters{{Fn|50}} (9’10") tall and weighs 500-1,000 kilograms (1,100-2,200lbs). Due to the nature of the zygotes that make up Astartes [[Gene-Seed]], all Space Marines are male.{{Fn|52}} Space Marines live far longer than average humans. [[Dante]], one of if not the oldest currently active known non-[[Dreadnought]] Space Marines, is currently over 1,100 years old.{{Fn|54}}A former [[High Marshal]] of [[Black Templars]], [[Ludoldus]], appears to have lived even longer while he was still alive, having served for over 2000 years.{{fn|16b}}{{fn|16c}}. A member of the [[Salamanders]] from the days of the [[Great Crusade]], [[Gravius]], later became stranded in space and was not rediscovered for some 10,000 years. The Marine was still alive in an inactive state, though withered and frail to the point of warranting being given a merciful death upon rediscovery.{{Fn|57}} Some Space Marines may initially believe their memory to be eidetic, but even their most glorious memories as an Astartes might be forgotten over the centuries.{{Fn|69}} This all lends to the theory that Space Marines while subject to gradual aging are "functionally immortal", though since they all inevitably die in battle one day there is not enough sufficient data to come to any firm conclusions.{{Fn|58}}