Content-based validity can be defined as the evidence of the degree to which the items of an instrument represent the construct to be inferred. It is the initial and unavoidable step for evaluating other sources of validity. However, it is one of the least reported psychometric properties in development of health measurement scales. That is why it is considered a challenge that must be overcome with the recognition of its value. In this narrative review, the concept of content as evidence of validity is revised and methods for evaluation are synthesized with an algorithm.