The problem of augmenting the functionality of a telephone system with new features, without causing unwanted interactions between the features, has received much attention during the last few years [BDCG89]. In industrial practice, detection is done not only by analyzing possible conflicts at the design stage, but also by running extensive libraries of test cases against the new system to see that it still behaves properly. Interestingly, the method we propose in this paper is similar to the one just described. However, being formal, our method allows precise reasoning, leading to precise criteria for choosing the set of test cases and analyzing the test results. Also, by relating the feature interaction problem to the well-known conformance testing problem, our approach makes available in this new area a wealth of well-established results.Because of space considerations, we consider only features that are defined independently and do not "build" on each other, meaning that these features make no assumptions about the behaviours of other features in the system. Also, we consider only single element features [CGLN94]. In this context, the main idea of our method is that in a system integrating features, the behavior of each feature (which is characterized as the sequences of observable actions generated by the feature) should be the same as its behavior in a system where all features are allowed to execute independently. The fact that this is not always the case is one of the main reasons of interactions in practice. In this framework, feature interactions can be detected at the specification stage by using test cases obtained 1