Research on aging uses rigorous quantitative, qualitative, and mixed methods to answer questions about the experiences, causes, and consequences of aging for individuals, families, communities, and societies. The accumulation of knowledge and deepened understanding that come from research are an improvement over the everyday speculations that we naturally develop as we navigate our lives. Producing valid knowledge requires well-reasoned choices about how a study should be designed. While research in any field must be carefully planned and carried out according to well-established standards, gerontology researchers face some unique challenges. This chapter reviews the logic of research on aging, and summarizes some of the distinctive issues in the field, including the age-period-cohort problem, the multiple ways in which age can be conceptualized as an explanatory variable, and the importance of longitudinal studies.