Psychological flow has been measured in several areas to analyse to what extent users are engaged in particular tasks, and is relevant in the design of products like software, videogames, and eLearning courses. Although there is an unknown number of questionnaires for evaluating different aspects of psychological flow, the research problem faced in this paper is the analysis of the validity of these questionnaires, since it has only been probed for some of them. Thus, our goal is to synthesize the current evidence regarding validated questionnaires in the English language for psychological flow measurement by conducting a systematic review according to the PRISMA framework. As a result, we found a total number of 34 validated questionnaires to assess flow. The number of their items ranged from 3 to 66, while 63 different dimensions of optimal experiences were taken into consideration. Moreover, the contexts of use differed, including methods to assess flow intensity, prevalence, variations, proneness, metacognitions, in crowds, observed, as dimensions of autotelic personalities, or to differentiate flow from clutch states. As a consequence, this paper facilitates the selection of the questionnaires for research or applied aims, far beyond the classic dichotomy of prevalence–proneness. Moreover, we present a reinterpretation of the nine-dimensional scheme of flow in stages, and recommend future research for engineering and computer science.