The number of intensive longitudinal studies that investigate affective experiences at the within-person rather than the between-person level is rapidly increasing. This paradigmatic shift comes with new challenges, such as questions revolving around how to measure within-person affect variation or more fundamental questions about the reliability and validity of constructs at the within-person level. We provide a review of substantive research published in Emotion since 2005, which revealed that to date no consensus has been established on measurement instruments for assessing within-person affective experiences. Our review also showed that researchers who are interested in within-person affect variation sometimes rely on measurement instruments that were established at the between-person level, which we think should be reconsidered. Finally, reliability estimates of state variation have been developed but are not comprehensively reported in studies on within-person affect variation. The purpose of this article is therefore to alert the reader to these issues and to highlight relevant criteria for selecting items and measurement instruments when studying within-person affect variation in intensive longitudinal studies. We recommend establishing common standards for measuring within-person affect variation and drawing from a common pool of instruments, which would allow direct comparison of results across studies. (PsycInfo Database Record (c) 2020 APA, all rights reserved).