Individual differences in the ability to control attention are correlated with a wide range of important outcomes, from academic achievement and job performance to health behaviors and emotion regulation. Nevertheless, the theoretical nature of attention control as a cognitive construct has been the subject of heated debate, spurred on by psychometric issues that have stymied efforts to reliably measure differences in the ability to control attention. For theory to advance, our measures must improve. We introduce three efficient, reliable, and valid tests of attention control that each take less than 3 min to administer: Stroop Squared, Flanker Squared, and Simon Squared. Two studies (online and in-lab) comprising more than 600 participants demonstrate that the three "Squared" tasks have great internal consistency (avg. = .95) and test-retest reliability across sessions (avg. r = .67). Latent variable analyses revealed that the Squared tasks loaded highly on a common factor (avg. loading = .70), which was strongly correlated with an attention control factor based on established measures (avg. r = .81). Moreover, attention control correlated strongly with fluid intelligence, working memory capacity, and processing speed and helped explain their covariation. We found that the Squared attention control tasks accounted for 75% of the variance in multitasking ability at the latent level, and that fluid intelligence, attention control, and processing speed fully accounted for individual differences in multitasking ability. Our results suggest that Stroop Squared, Flanker Squared, and Simon Squared are reliable and valid measures of attention control. The tasks are freely available online: https://osf.io/7q598/. Public Significance Statement Reliably measuring individual differences in attention control has posed a challenge for the field. This paper reports the development and validation of three 90-s tests of attention control, dubbed the "Squared" tasks: Stroop Squared, Flanker Squared, and Simon Squared. The three Squared tasks demonstrated great internal consistency reliability and test-retest reliability, strong evidence for convergent validity with other measures of attention control, and explained a majority of the positive manifold and variance in multitasking ability. The three Squared tasks can be administered online via web browser, E-Prime, or as standalone programs for Mac and Windows (https://osf.io/7q598/). The three Squared tasks demonstrate that it is possible to reliably measure attention control at the observed and latent level by avoiding the use of response time difference scores. Furthermore, the measures reveal that individual differences in attention control can be represented as a unitary latent factor that is highly correlated with complex cognitive task performance.