A new Living Textbook topic chapter, “Electronic Health Records-Based Phenotyping,” has just been published. The chapter defines computable phenotypes and describes their role in data queries of electronic health records as part of pragmatic clinical trials. A main focus of the chapter is outlining mechanisms for identifying and evaluating phenotype definitions, with particular emphasis on standardization efforts of the NIH Collaboratory, including the Table 1 Project. Also included are links to recommended phenotype definitions from the Collaboratory Phenotypes, Data Standards, and Data Quality Core.