The purpose of this paper is to make an appraisal of the new questions that are being raised by on-line services with regard to the measurement of their audience and site visits. It will also aim to provide concrete answers in terms of systems, indicators and methods. To have a fuller understanding of the challenges involved in measuring on-line services, it is important first to understand and appreciate the innovations they bring compared with conventional media, both with regard to the original possibilities they offer and the unique behaviour patterns of those who use them. It is these new dimensions of the medium and these alone which are leading to changes and shifts in the forms that advertising takes at present. They result in creating new needs in terms of measurement and therefore in the development of new methods. All these issues are covered in this paper, whose author has been leading a working group at Mediametrie over the past few months composed of the major players in the profession in France. The observations and conclusions that follow are the result of this collective thinking.