Motivation is a fundamental concept in understanding people’s experiences and behavior. Yet, motivation to engage with an interactive system has received only limited attention in HCI. We report the development and validation of the User Motivation Inventory (UMI). The UMI is an 18-item multidimensional measure of motivation, rooted in self-determination theory (SDT). It is designed to measure intrinsic motivation, integrated, identified, introjected, and external regulation, as well as amotivation. Results of two studies (total N = 941) confirm the six-factor structure of the UMI with high reliability, as well as convergent and discriminant validity of each subscale. Relationships with core concepts such as need satisfaction, vitality, and usability were studied. Additionally, the UMI was found to detect differences in motivation for people who consider abandoning a technology compared to those who do not question their use. The central role of motivation in users’ behavior and experience is discussed.