Although the implementation of an ERP system is known as being a hard, hazardous and difficult task, most of the communication on the subject focuses on - and generalises from - success stories. We have chosen here to discuss several difficult implementations of ERP systems made in France within the next five years. The context of these case studies is explained, the difficulties which have been encountered are listed and tentative explanations are provided. We finally emphasise some basic and pragmatic principles which should remain in mind while installing such systems.