Learning, the acquisition of new knowledge or behavior due to environmental stimulation, is a defining characteristic of almost all living things from unicellular organisms to fully neural multicellular vertebrates. However, much of what is currently known about organismal learning, memory-formation, and behavior is focused entirely on neural organisms. However, learning does not inherently require multicellularity. The work of David Wood with Stentor coeruleus demonstrated that unicellular organisms are capable of non-associative learning including habituation. Herbert Jennings showed in Stentor roeseli the presence of a hierarchical behavior tree from which the stentor appears to choose responses to chemical repellents. And finally, Beatrice Gelber showed that Paramecium aurelia is capable of associative learning through classical conditioning of a food-based response. But what can be gleaned from studying how unicellular organisms learn? First, the mechanisms of neural organism learning distill down to changes in single cells. Therefore, insights into how unicellular organisms change as they acquire new knowledge or behavior could improve our general understanding of the mechanisms of learning. Second, how learning has evolved over time in different organisms is an interesting question that can be partly approached by defining what forms of learning are universal and to what extent they are conserved between neural and unicellular organisms.