- "No ordinary man can a Captain be, for many are the paths to victory and he must be master of them all. His very being shall be an extension of the Emperor's work. With every strike of his sword, with every word of his speech does he reaffirm the ideals of our honoured master. He will vanquish darkness and heresy with every thought, word and deed. So shall his coming be a sign of deliverance to the dutiful and a herald of dismay to all traitors. No living man shall stay his wrath."
- — From the teachings of Roboute Guilliman, the Apocrypha of Skaros
A Captain is a senior Astartes officer who commands each of the 10 companies in a standard Space Marine Chapter that follows the dictates of the Codex Astartes. These hardened veterans have proven themselves in the maelstrom of battle as members of their Chapter's elite 1st Company, or through their bloody and dedicated service in their own company prior to the death of their predecessor and their ascension to command. As penultimate masters of the battlefield, Captains are able to read its ebb and flow, and adjust their battlefield formations accordingly. Leading from the front, by his example shall a Captain's men know what it is to be an Adeptus Astartes, and from his teachings shall they learn the trade of battle in the Emperor's name.
Each Captain is a hardened Veteran Astartes, a master strategist who has proven his prowess in battle as a member of the Chapter's elite 1st Company, or through distinguished service in his own company as a squad sergeant prior to the death of his predecessor. In addition to his Chapter rank, each Captain also bears one or more honorific titles associated with the command of a particular company or responsibility. Some, such as Master of the Watch and Master of the Recruits, are common titles in use across almost all Space Marine Chapters. Other titles are products of a particular Chapter's history, culture and military specialties. The Captain of the White Scars' 4th Company is traditionally the Master of the Hunt, while the Aurora Talons Chapter, specialising in space warfare, maintains no less than five Master Bombardiers among their company Captains.
Individual Captains can also be assigned to command missions that require the deployment of elements of the Chapter beyond their own company. In these situations an individual Captain takes on the temporary rank of Force Commander and gains precedence over even other Captains assigned to this mission. It is not enough for an Astartes Captain to be a skilled warrior -- each Chapter boasts many such men amongst its roster -- he must also have a superhuman grasp of military strategy and combat tactics, as well as the intelligence to employ them in the ever-changing arena of warfare. It is commonly said in the Imperium that in terms of raw military might, every Space Marine is easily worth a dozen or so Imperial Guardsmen. Under the command of an experienced Captain, this value can swell tenfold.
A Space Marine Captain is not simply a master of warfare, he must also have the gift of diplomacy and political adoitness. While most Imperial Planetary Governors and Commanders are more than happy to receive aid from the Space Marines when in trouble, there are those whose arrogance must be carefully negotiated in order to prevent a battle's collapse into a true disaster for the broader Imperium through unnecessary political infighting. This is not to say a Space Marine Captain lacks for ultimate authority when dealing with hubris and failure to the Emperor -- as Captain Kayvaan Shrike of the Raven Guard's 3rd Company once proved by executing a vacillating Lord Cardinal Dostok on the spot. That said, subtler methods of diplomacy are best employed in grim situations -- the necessary punitive measures can be enacted after the threat has been ended. A Space Marine never bends the knee lightly to any non-Astartes, but the needs of the Emperor's realm must override any personal feelings. The mission is always all. Humanity shall be saved from its own petty failings, even if this must be done one world at a time.
- Codex: Space Marines (5th Edition), p. 54