Informed by identity literature, this study develops a set of theory-based definitions and measures for investigating how individuals' self-identification with particular IT (termed IT identity) influences their ongoing IT use behaviors. In doing so, it contributes to both IS theory and research methods. For methods, determining whether a construct can be measured by at least two methods is an often overlooked aspect of establishing construct validity. Detailing the steps involved in CFA-based MTMM analysis may serve as a guide to IS researchers seeking to develop valid, or evaluate alternative, measures of perceptual phenomena. For IS theory, conceptualizing IT identity as a new form of material identity has the potential to open up new ways of thinking about individuals' long-term interactions with IT. In clearly articulating the construct's conceptual domain and operationalizing the construct, this study reinforces the point that there is a lot of theory involved in measurement.