Many of the real-world data sources used in clinical research are considered "secondary" sources, because the data were collected for a purpose other than the research project for which they are being used (eg, billing or clinical care). This contrasts with primary data sources, where the data are captured specifically for clinical care, billing, or a specific prospective study. (See the Acquiring Real-World Data chapter of the Living Textbook for more information about the different types of real-world data.) Consequently, before a real-world data source can be used in an analysis, one must understand its characteristics and limitations to determine whether it can be appropriately used to answer the question at hand. A given dataset may be well suited to answer a research question for a specific patient population over a certain time period, but not suitable for a different population or time frame. For this reason, data must be assessed to determine whether they are fit for their intended use or purpose prior to their use in research settings. This chapter describes several approaches that can be used to facilitate such assessments.