Paleontologists, scientists who study the history of life on Earth, work in a dynamic area of science. Think of putting together a jigsaw puzzle with most of the pieces missing—that’s what creating the fossil record is like. Each time a new piece is discovered; ideas about the whole picture become clearer (and sometimes even change considerably). This chapter focuses on how we know what we know about prehistoric life, how scientists’ ideas have changed over time, and what mysteries of the past remain to be solved.