Previous work in the field of substance use/misuse has suggested that client beliefs about addiction are quite diverse, and may influence the effectiveness of various treatment approaches. This paper presents an Addiction Belief Inventory (ABI) that was developed and evaluated to assess personal beliefs about addiction and substance use problems. The ABI is a 40-item instrument developed using two clinical samples: an alcohol user treatment group (N = 134) and a dual diagnosis treatment group (N = 536). Confirmatory factor analysis revealed seven stable subscales: inability to control, chronic disease, reliance on experts, responsibility for actions, responsibility for recovery, genetic basis, and coping. Multivariate analyses provided preliminary reliability and validational support. The utility of the ABI for clinical and research purposes is discussed, along with suggestions for future research and improvements to the instrument.