The level and nature of discipline in modern political parties remain two of the most fascinating aspects of politics. This study provides a review of the literature on the role of political parties and the perceived need for some form of collective action within parliament. A survey of work on party discipline in other countries is also provided, followed by a closer examination of what is meant by ‘discipline’ . We explore the level of cohesion and discipline in Irish parties. The organisation of Irish parties and the development of the whip system is examined along with other influences on the behaviour of TDs, such as the electoral system and Irish political culture. A systematic account and analysis of cases of indiscipline between 1969 and 1997 is provided and analysed. The work concludes with a social-psychological explanation of TDs behaviour which is tested using available empirical evidence and a laboratory experiment. The thesis argues that the high levels of discipline among members of Irish parliamentary parties is best explained by reference to rational choice theories of political behaviour.