This paper examines the determinants of household car ownership in Ireland, using longitudinal data for the period 1995-2001. This was a period of rapid economic and social change in Ireland, with the proportion of households with one or more cars growing from 74.6 per cent to 80.8 per cent over the period. Understanding the determinants of household car ownership, a key determinant of household travel behaviour more generally, is particularly important in the context of current policy developments which seek to encourage more sustainable means of travel. In this paper, we exploit the availability of longitudinal data to estimate dynamic models of household car ownership, controlling for unobserved heterogeneity and state dependence. We find income and previous car ownership to be the strongest determinants of differences in household car ownership. Other important influences include household composition (in particular, the presence of young children) and lifecycle effects, which create further challenges for policymakers in seeking to change travel behaviour.