Species were unique lifeforms. They were generally divided into three classifications based on their level of sentience: sentient, semi-sentient, and non-sentient. There were over 20 million sentient species known to the galaxy. Sentient species were sometimes erroneously referred to as races, which more accurately referred to groups or subspecies within a species.

Many institutions in the galaxy focused on the study of sentient species. The most notable were the University of Coruscant and the University of Sanbra. Those scientists and anthropologists that studied sentient species were known as sentientologists.

One of the major works on sentientology, Obo Rin‘s Catalog of Intelligent Life in the Galaxy, was commissioned by Lord Vader, and was carried out under the auspices of Major Vontenn, Liaison Officer of Sentiology Studies. While it contained much information, it was also a conduit for Imperial propaganda, and as such, had a tendency to depict some species as less developed than was actually the case. Rin’s credentials as a scientist were also questionable.

Another major work on sentientology was The University of Sanbra Guide to Intelligent Life by the Iyra Tem Eliss. Rather than a scientific study, it was designed to facilitate first contact with species, providing «guidelines as to common cultural interests, goals, fears and drives.» However, it departed from Imperial doctrine on a number of points. The Empire tried to apprehend its author shortly before publication, but the academic escaped and released his research through underground networks and NewsNets.

The Shi’ido researcher Mammon Hoole wrote the acclaimed The Essential Guide to Alien Species. He gained unique insights into species by infiltration using his shapeshifting abilities. Other documents that detailed sentient species included the Universal Encyclopedia.