Book excerpt: Explaining the nature of theories in the field of child development (including why theories are useful and why there are multiple theories), Thomas' highly respected book covers the widest range of theories of any text currently available. Using the following criteria: "Has the theory been popular?," "Has the theory been influential?," and "Is the theory representative of a class or family of theories?," Thomas identifies the goals and describes the findings of more than two dozen theorists and inspects a series of child development issues that bridge a variety of theories. Additionally, Thomas suggests a variety of ways to compare theories and existing explanations of how a child develops.