Many individuals have had the experience of “coming to” in the middle of a task and realizing that they aren’t paying attention. This shift in attention away from a primary task is known as mind-wandering. There has been an increasing amount of research on mind-wandering, but results are inconsistent. In this thesis, we investigate two sources of inconsistency: differences in the way mind-wandering is measured (thought-probes, post-task reports), and differences in the type of mind-wandering experienced (intentional, unintentional) under different task conditions. Because there is a danger that mind-wandering may also vary depending on the task, we investigated these issues in two primary tasks: a simulated driving task, and the N-back task (a working memory task used in studies of mind-wandering). In Experiments 1 and 2, we manipulated how mind-wandering was measured (thought-probes, post-task reports) and examined effects on rates of mind-wandering changes in task performance, finding that measurement type had an effect on both. Although post-task reports may be less sensitive and capture less mind-wandering, thought-probes interrupt the flow of the task, changing performance and influencing the way mind-wandering varies with time on task. This suggest that some discrepancies emerge because the two report types do not measure the same thing and can affect how other variables influence performance. Our second line of research investigated the prevalence of intentional versus unintentional mind-wandering (assessed by thought-probes), as it as it varies by task difficulty (Experiment 3: driving, Experiment 4: N-back task). Here we found that unintentional mind-wandering was more prevalent than intentional and more detrimental to primary task performance—especially when the task was difficult. Across all experiments, we also monitored the relationship between mind-wandering and differences in working memory (measured by the Operation Span) and sustained attention (measured by the Sustained Attention to Response Task), finding that effects varied across studies. Overall, this research challenges the assumption that mind-wandering is the same regardless of context (primary task), suggesting that researchers should be cautions in generalizing across primary tasks when studying mind-wandering. These findings have ramifications for theory and methodology in the mind-wandering literature, as well as implications for driver safety.