In this chapter I provide a brief overview of the scientific method, including discussions of random sampling, random assignment, internal and external validity, and distinctions between correlational and experimental research with applications to the study of gender. I then discuss the specific challenges researchers face in the study of gender, including biases on the part of the researcher and the participant, variables that are confounded with sex, and the impact of situations on behavior. Afterward I turn to a history of the scientific study of gender, which begins with a study of the brain to identify the intellectual differences between men and women and then a shift to the psychological characteristics that might differentiate women and men. The latter movement focused on personality traits that vary in men and women, originally conceptualized as masculinity and femininity but more recently conceptualized as agency and communion. The chapter ends with a discussion of the most recent transformation in the study of gender from sex as a target variable to sex as a social category. Research on gender-role strain is discussed.