Intelligence in humans and other animals has been measured and defined by flexibility in problem-solving, learning, memory, reasoning, abstract thinking, planning, and communication and language, and is often associated with increased brain size. While growing and maintaining a large brain is metabolically costly, larger brains and increased intelligence have evolved in the Primate lineage. In this chapter, we discuss the theories behind the evolution of intelligence in primates, describe the methods used to study intelligence in non-verbal subjects, explore the physical and social cognitive abilities of nonhuman primates, and detail the value of the comparative approach to the study of intelligence.