Despite evidence that it exists, short-term within-individual variability in cognitive performance has largely been ignored as a meaningful component of human cognitive ability. In this article, we build a case for why this within-individual variability should not be viewed as mere measurement error and why it should be construed as a meaningful component of an individual’s cognitive abilities. We argue that in a demanding and rapidly changing modern world, between-individual analysis of single-occasion cognitive test scores does not account for the full range of within-individual cognitive performance variation that is implicated in successful typical cognitive performance. We propose that short-term repeated-measures paradigms (e.g., the experience sampling method (ESM)) be used to develop a process account of why individuals with similar cognitive ability scores differ in their actual performance in typical environments. Finally, we outline considerations for researchers when adapting this paradigm for cognitive assessment and present some initial findings from two studies in our lab that piloted the use of ESM to assess within-individual cognitive performance variation.