Originally prepared to provide background information and analysis for a Canadian Labour Congress conference on rebuilding unions in the Fall of 2003, this article maps trends in union density i.e., the proportion of workers covered by a collective agreement.  It provides a short overview of the period since the mid 1980s, and a much more detailed analysis of the period from 1997 to 2002 for which detailed data from Statistics Canada's Labour Force Survey are available. Union density has trended down in the private sector to below 20% more because of a density decline among male blue collar workers than because of deindustrialization per se. Union density has remained high in public and social services, including outside the direct public sector, and this mainly explains why union density has held up much better among women than men. Union density is low, but relatively stable, in private consumer and business services. Density has fallen proportionately less in the higher union density provinces, particularly Quebec, and has fallen to well below average levels among private sector workers in the fastest growing provinces of Ontario and Alberta.