The availability of secondary data sources is growing in the field of public health, as is the appeal of conducting research using already collected data. When formulating a research plan that utilizes secondary data, however, researchers need to make several considerations that are unique to analyzing secondary data. Using an example from the author’s work, “Sensitive Periods for the Association between Childhood Maltreatment and BMI,” this article illustrates the decision points and challenges that arise with secondary data analysis. The author posits that there are three main components of developing a secondary data research plan: (a) defining the research aim, (b) selecting the dataset, and (c) determining the analysis method. When researchers thoughtfully consider each of these components, secondary data research can explore novel hypotheses and build the groundwork for future studies.