Political parties in New Zealand are now affected by elements of ideological erosion - they are characterised by both a policy convergence and a general electoral pragmatism. This thesis attempts to characterise and explain this ideological erosion in New Zealand party politics. It also aims to show that the erosion of ideology is closely related to a host of other aspects of party transformation, such as weakened partisan ties (including the decline in party membership, decline in linkages with interest groups, and class dealignment), as well as an increased reliance on the state for resources, the professionalization of the party organisations, and an increased anti-party sentiment in society. The central argument of this thesis is that these phenomena relate closely to and reflect the shift away from the 'mass membership' type of political party to an 'electoral-professional' model. It is argued that this transition has been in motion since the 1950s, but accelerated in the 1970s and then again in the 1990s. This debate revolves around a paradox in which, on the one hand, political parties in advanced industrial countries remain central to the conduct of parliamentary democracy and, on the other, they often seem to be less connected to the constituencies they claim to represent and less able to provide voters with effective choices. This thesis engages with this debate by examining both ideological and organisational transformation in one particular democracy, New Zealand.