3 Sarah Jane Delany Pádraig Cunningham Lorcan Coyle 2004 Lorraine McGinty and Brian Crean Proceedings of the Fifteenth Irish Conference on Artificial Intelligence and Cognitive Science (AICS'2004) Castlebar, Mayo, Ireland 9-18 08/09/2004 case based reasoning, spam filtering, ecue Because of the changing nature of spam, a spam filtering system that uses machine learning will need to be dynamic. This suggests that a case-based (memory-based) approach may work well. Case-Based Reasoning (CBR) is a lazy approach to machine learning where induction is delayed to run time. This means that the case base can be updated continuously and new training data is immediately available to the induction process. In this paper we present a detailed description of such a system called ECUE and evaluate design decisions concerning the case representation. We compare its performance with an alternative system that uses Naïve Bayes (NB). We find that there is little to choose between the two alternatives in cross-validation tests on data sets. However, ECUE does appear to have some advantages in tracking concept drift over time.