Mobile devices are often described as very personal objects and users are said to develop deep relationships with them. Motivated by such recurring statements, this paper introduces user-device attachment as a concept describing the relationship between users and their mobile devices. By drawing on literature from marketing, HCI, and sociology, the domain and dimensions of userdevice attachment are described. Three dimensions: symbolism, aesthetics, and perceived necessity are conceptualized to represent elements of user-device attachment. An initial instrument for measurement is developed and assessed with exploratory and confirmatory factor analyses. Corresponding with the conceptualized dimensions, a three-factor solution was found and confirmed using data gathered in two surveys (n1=130, n2=323). The resulting instrument provides a basis for future discussions and conceptual and empirical work on the phenomenon of user-device attachment in mobile user behavior. Possible benefits of applying the construct to areas like mobile advertising and mobile service development are identified.