Rating of perceived effort (RPE) scales are the most frequently used single-item scales in exercise science. They offer an easy and useful way to monitor and prescribe exercise intensity. However, RPE scales suffer from methodological limitations stemming from multiple perceived effort definitions and measurement strategies. In the present review, we attend to these issues by covering (1) popular perceived effort definitions, (2) the terms included within these definitions and the reasons they can impede validity, and (3) the problems associated with using different effort scales and instructions. We pose that the large number of interactions between definitions, scales, instructions and application strategies, threatens measurement validity of RPE. We suggest overcoming these limitations by narrowing the number of definitions of perceived effort, the number of terms included within them, and the number of scales and instructions used. By following these recommendations, we expect the field will increase measurement validity and become more comprehensive.