There is an estimated need of 33,000 units of supportive housing currently in Ontario. This technical report provides preliminary population-based estimates of need for housing with support for people with serious mental illness or addiction in Ontario. It has generated an estimate of the need for housing with support for mental illness and addictions. This is based on cascading prevalence and propensities of severe mental illness, household formation, low income, need for support in this subset, and bearing in mind the population already in housing with support (or in institutions and not ready).