This chapter provides the subfields of gerontology and their unique contributions to the study of aging. It describes the importance of theory and gives the two levels of theory used in gerontology. The chapter also provides three major perspectives gerontologists used in research. It contains pros and cons of cross-sectional and longitudinal research designs. The chapter explains the strengths and weaknesses of quantitative and qualitative research methods. It discusses the importance of information literacy in the age of the Internet and online research. The chapter presents the examples of ethical issues researchers face and how they deal with the issues. It also explains the importance of applying research to improve older peoples' lives. The theories focus on large-scale events, such as historical changes in family size and structure, health care policies, and how industrial or agricultural societies treat their older people.