This chapter describes research in comparative psychology. Many of these studies aimed to show that species differ in their learning and problem-solving abilities. The chapter starts with studies that attempted to teach a language to chimpanzees and examined their apparently remarkable ability to learn by imitation. The chapter then describes studies of complex learning in monkeys, focusing on the work of Harlow, before moving on to dolphins. The attempt by Bitterman to demonstrate differences between rat-like and fish-like learning abilities is followed by an account of experiments that used matching-to-sample tasks in pigeons and compared the learning abilities of corvids with those of pigeons.