Shelter is the biggest expenditure most households make and its affordability can have an impact on wellbeing. For this reason, housing affordability is closely watched by a wide range of stakeholders—from housing advocates to policy analysts—interested in housing and the broader welfare of Canadians. Measuring affordability involves comparing housing costs to a household’s ability to meet them. One common measure is the shelter cost-to-income ratio (STIR). The 30% level is commonly accepted as the upper limit for affordable housing. Those who spend 30% or more have been, and continue to be, the subject of intense study—do they do so out of choice, having the means and preference to spend more than the norm; or out of necessity, having low income and possibly being in housing need. Housing affordability is also a critical input to Canada Mortgage and Housing Corporation’s (CMHC) core housing need indicator.1 The core housing need indicator “identifies those households unable to obtain market housing that is in adequate condition, of suitable size and, at the same time, affordable” (CMHC 1991). The information is used by governments to help design, deliver, fund and evaluate social housing programs. Up to now, STIRs have described affordability at a particular point in time. No source followed households over time, collecting both incomes and shelter costs. While Statistics Canada’s Survey of Labour and Income Dynamics (SLID) provides household income over a six-year period, it does not normally collect data on shelter costs. So, for the last five years, CMHC has sponsored a module of housing questions and now this information enables a first-ever longitudinal review of housing affordability. Cross-sectional estimates indicate that around one-fifth of Canadians lived in households spending more than the affordability benchmark in any one year between 2002 and 2004. Longitudinally, however, less than one tenth lived in households that persistently spent above the benchmark between 2002 and 2004.