Inherent to commitment research is the need to test change or differences in attitudes between groups or across time. A fundamental assumption of these tests is that commitment measures assess constructs equivalently, or are invariant, between groups or across time periods. Understanding how to test this assumption not only helps determine the quality of commitment measures, it also offers opportunities for testing substantively meaningful research questions. Due to the importance of this assumption to commitment research, this chapter revisits and updates Vandenberg and Lance’s (2000) seminal summary of the recommended measurement equivalence or invariance (ME/I) testing steps using confirmatory factor analysis. The chapter also alerts commitment researchers to easily overlooked, yet important issues related to ME/I testing, citing specific examples from the commitment literature as illustrations. Finally, a discussion of ME/I testing using item response theory is provided for comparative purposes. Reference: Vandenberg, R.J. and Lance, C.E. (2000). A review and synthesis of the measurement invariance literature: Suggestions, practices and recommendations for organizational research. Organizational Research Methods, 3, 4_70.