Objective: The Conners Continuous Performance Test-3 (CCPT-3) is a computerized test of attention frequently used in clinical neuropsychology. In the present factor analysis, we seek to assess the factor structure of the CCPT-3 and evaluate the suggested dimensions in the CCPT-3 Manual. Method: Data from a mixed clinical sample of 931 adults referred for neuropsychological assessment across four centers were analyzed. Nine standard and eight experimental measures were subjected to an exploratory factor analysis to evaluate factor models ranging from one to six factors. Results: The analysis supported a four-factor model with one overall attention factor and three factors of distinct mechanisms underlying inattention: impulsivity, vigilance, and sustained attention. This closely aligns with the four dimensions outlined in the CCPT-3 Technical Manual and the factor analyses from the CCPT-II. There were some differences between the four-factor model and the interpretations recommended in the Technical Manual. Perseverations were associated with the inattention factor rather than the impulsivity factor, and reaction time was exclusively linked to impulsivity. Incorporating error measures into the vigilance factor suggests that decreases in responsivity, rather than decreases in correct responses, underpin vigilance decrements. Including response bias by inter-stimulus interval (ISI) and by blocks in the analysis indicates that a decrease in arousal may also explain impairments in sustained attention. Conclusion: This study supports the notion in the Technical Manual that CCPT-3 measures both overall attention and three different mechanisms that mediate inattention: impulsivity, vigilance and sustained attention.