Although the personality-performance relationship has been studied extensively, most studies focused on the relationship between between-person differences in the Big Five personality dimensions and between-person differences in job performance. The current paper extends this research in two ways. First, we build on core self-evaluations (CSEs): an alternative, broad personality dimension that has proven to be a good predictor of job performance. Second, we tested concurrent and lagged within-person relationships between CSEs and task performance, organizational citizenship behaviour (OCB), and counterproductive work behaviour (CWB). To this end, we conducted two experience sampling studies; the first one assessing the relationship between state CSEs and levels of momentary task performance and OCB, and a second study in which employees reported on their level of state CSEs and momentary CWB. Results showed that there is substantial within-person variability in CSEs and that these within-person fluctuations relate to within-person variation in task performance, OCB, and CWB towards the organization, and CWB towards the individual. Moreover, CSEs prospectively predicted within-person differences in task performance and CWB towards the organization, whereas the reversed effect did not hold. These findings tentatively suggest that state CSEs predict performance, rather than the other way around.Keywords: state core self-evaluations; task performance; organizational citizenship behaviour; counterproductive work behaviour; within-person variabilityIn the domain of work and organizational psychology, the personality-performance relationship has been studied extensively (Barrick & Mount, 1991;Barrick, Mount, & Judge, 2001). The primary reason is that predicting employee performance from stable, person-related characteristics (i.e., personality traits) is attractive from both a practical and a theoretical point of view. With respect to the former, a strong, reliable relationship between personality and performance paves the way for using parsimonious personality assessment instruments in, for example, employee selection procedures. Regarding the latter, it yields useful insights into the determinants of job performance and the consequences of personality at work.Up until now, most studies on the personality-performance link have focused on how individual differences on each of the Big Five personality dimensions related to individual differences in general indices of work performance (Barrick & Mount, 1991;Hurtz & Donovan, 2000;Neal, Yeo, Koy, & Xiao, 2012). Recently, however, this approach has been called into question. The first reason is that awareness is growing that performance is not static, but rather episodic in nature (Beal, Weiss, Barros, & MacDermid, 2005;Debusscher, Hofmans, & De Fruyt, 2014, 2015, which means that it not only varies between individuals, but also changes over time within an individual. As a result, it becomes important to study not only the stable, trait-like antecedents of performance, but ...