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In clinical practice and population health, it is common to use questionnaires to assess conditions or variables that are not directly observable. However, the construction and validation of these instruments or questionnaires are often poorly understood. This narrative review aims to summarize in a general way the process of construction and validation of these questionnaires in order to have a better understanding of this process, the aspects that are evaluated, and the best way to use them. The validation of questionnaires corresponds to a process of analysis of the questionnaire, aiming to measure a latent variable and its dimensions, which cannot be observed directly. A latent variable can be inferred through a set of specific attributes that are part of it, such as the items of a questionnaire, which are observable. Through a narrative review, this article addresses the fundamental concepts of questionnaire or test validation, latent variables or constructs, reliability and validity studies, and the factors that theoretically affect the latter two characteristics. Examples of these concepts are presented in the text., Competing Interests: The authors completed the ICMJE conflict of interest statement. They declared that they received no funding to complete this article, have no financial relationships with organizations that may have an interest in the published article in the last three years, and have no other relationships or activities that may influence the publication of the article., (This work is licensed under a Creative Commons Attribution 4.0 International License.)