This paper illustrates the use of discourse maps as a research tool to represent patterns of interaction in email discussions. Developed from an idea by Black et al.(1983), the discourse map can represent visually the complex interaction of an email discussion list, showing patterns of speaker activity, the distribution of turns across parallel topics, and the spread of contributions over time. This method of representation facilitates the identification of significant features in the discourse, such as consecutive turns by the same participant, multiple topics within a single message, and topics which fail to generate a response. Discourse maps also facilitate comparison of patterns of interaction in different discussion groups. Data samples taken from different discussions can be seen to differ in several respects, such as the number of topics that fail to get a response, and the level of participation in individual topics. This technique has been developed in order to represent email discussions, but it can also usefully be applied to other forms of discussion in different media for the purpose of comparison. This paper uses discourse maps to compare different email discussions, and also to illustrate the different interaction patterns of a sample of newspaper letters and a radio discussion.