Tests of working memory capacity (WMC) and fluid intelligence (gF) are thought to capture variability in a crucial cognitive capacity that is broadly predictive of success, yet pinpointing the exact nature of this capacity is an area of ongoing controversy. We propose that mind-wandering is associated with performance on tests of WMC and gF, thereby partially explaining both the reliable correlations between these tests and their broad predictive utility. Existing evidence indicates that both WMC and gF are correlated with performance on tasks of attention, yet more decisive evidence requires an assessment of the role of attention and, in particular, mind-wandering during performance of these tests. Four studies employing complementary methodological designs embedded thought sampling into tests of general aptitude and determined that mind-wandering was consistently associated with worse performance on these measures. Collectively, these studies implicate the capacity to avoid mind-wandering during demanding tasks as a potentially important source of success on measures of general aptitude, while also raising important questions about whether the previously documented relationship between WMC and mind-wandering can be exclusively attributed to executive failures preceding mind-wandering (McVay & Kane, 2010b). (Contains 4 figures, 3 tables and 5 footnotes.)