This paper uses Lee's (1978) model to determine the wage gains associated with trade union membership and the reasons individuals join trade unions in Australia The data are derived from the 1984 Australian National Social Science Survey. A major conclusion is that unions secure a 17.22 per cent wage gain for their members, other things the same. This expected wage premium is shown to have an important positive impact upon the union membership decision. The welfare loss associated with the union wage premium is argued to be relatively minor about one half of one per cent of gross domestic product