Humanity is by far the most populous species in the galaxy, as they have been for uncounted years now. Although human population is nowhere near what it was during the golden years of the Dark Age of Technology, it still outstrips any alien population by a large factor. Even the Orks, for all their numbers, would be but a but a drop in the vast pool of humanity. Indeed, some scholars speculate that humanity outnumbers the total alien population of the galaxy, through by how much is pure speculation.

Baseline humans: Humans come in a bewildering array of shapes and sizes, but they are all share a common ancestry and can freely interbreed with one another (one of the Adeptus Terra's hallmarks for defining 'humanity'). The greater variety within the human genome in the 41st Millennium is a result of extensive geneering in the distant past, natural evolution over as many as 2.000 generations, and the relative isolation of human populations. Prior to the advent of the warp-drive human colonies were effectively isolated. Travel was likely much more common during the Dark Age of Technology, which resulted in the human gene-pool being reunited – for a time. In the Age of Strife human worlds (those that survived) were again isolated, and even in the Age of the Imperium this largely holds true; since travel between world is not a readily available option for the masses each planetary gene-pool tends to be relatively self-contained, and the local population tends to take on some common characteristics over the course of millennia (but there is bound to be great variations within the planetary gene-pool, as various regions and hives tend to keep to themselves). Mass migrations take place on occasion, to colonize new worlds or re-populated devastated ones, or as the result of Crusades and other monumental affairs. But within a few thousand years the new once-diverse gene-pools have become as calcified as their forebears.

Ab-humans: Back in the ancient days of the Stone Men genetic engineering was routinely used to adapt the colonists to their new homes (above and beyond the common geneering applied to all humans of that advanced age). Later generations of colonists would frequently receive further alterations to make their lives easier; on a high-gravity world the colonists might be geneered short, stocky, and with heavy musculature (eventually leading to the squat strain of ab-human). The most common ab-human types found in M41 are squats (endomorphs), ratlings (small ectomorphs), feylings (tall ectomorphs), and ogryns (large mesomorphs). Squats are short and powerful, ratlings are short and lean, feylings are tall and thin, while ogryns are tall and very powerfully built. Beyond this generalization great variation exists, as it does within the ranks of 'common' man; ratlings, for example, evolved on many different Hive Worlds during the Age of Strife, with each Hive World producing its own subtle variations. Certain other ab-human strains do exist and are recognized as being fully human, but are generally confined to a single world or small area of Imperial Space.

Beast-men: The term 'beast-men' was coined by the Ordo Xenos as a catchall for ab-human strains considered too far removed from the rest of humanity to be considered fully human. Whatever their shape (some have fur to protect them from cold, others have gills and webbed extremities to allow them to live underwater) they come from human gene-stock (they are not xenos as such), their mutation is stable and they can interbreed with other human strains as well as their own kind. Some beast-man strains are (barely) tolerated as long as they remain useful to the Imperium; some being allowed to serve as auxiliaries to the Imperial Guard for example. Others strains are marked for extinction as soon as the Imperium makes contact with them. Indeed, the worst cases are able to interbreed only with others of their kind and/or bear little resemblance to normal humans. Such degenerate creatures are pushing the boundaries of what it means it be human and must be purged.

Mutants: Humanity is constantly evolving, and the extreme numbers and conditions under which much of humanity lives provides much impetus for evolutionary mutation. The presence and corrupting properties of [classified] also cannot be discounted, and adds to both the rate of mutation and the severity of it. Mutants come from human genetic stock, and the level of mutation decides if they can be classified as human or not. In theory all mutation is persecuted by the Imperium, but in practice the teeming masses as left alone with their mutations unless they pose a risk to society or if their abomination is too plain (in which case they had better hide or suddenly find themselves stalked by the Ordo Hereticus).