Despite housing stability being a key concept in housing and homelessness policy, research, and service provision, it remains poorly defined and conceptualized, and to date there are no standard measures. We use in-depth qualitative interviews with 51 young people transitioning away homelessness over the course of a year to examine the core dimensions of housing stability. Due to the potential for sudden change, we define housing stability as the extent to which an individual's customary access to housing of reasonable quality is secure. We define housing security among 8 main dimensions: housing type, recent housing history, current housing tenure, financial status, standing in the legal system, education and employment status, harmful substance use, and subjective assessments of housing satisfaction and stability. Based on these dimensions, we suggest a brief 13-question scale that measures housing security.