The ANTARES collaboration is currently constructing a large area neutrino telescope for deployment in the Mediterranean Sea. The ANTARES detector will be located 30 km off the southern French coast. After introducing the detection principle, the details of the detector design are considered. In describing the ANTARES site, emphasis is placed on results from more than 25 deployments of autonomous mooring lines instrumented with various measuring devices. These deployments have led to an understanding of the water and environmental parameters at the site. A neutrino telescope can address a wide-ranging scientific programme which covers topics in particle physics and astrophysics, a brief summary of this programme is made, following which, aspects of the expected detector performance, specifically the angular and energy resolutions, are discussed. Finally, the results from a ``demonstrator string'' are presented. This is a full-scale, part-instrumented string which was deployed in the Mediterranean Sea in late 1999. The string has provided the first recorded down-going muons which have been subsequently reconstructed and analysed, examples of these events and their properties are given. The demonstrator was also equipped with the instrumentation required for determining the position, inclination and orientation of the string as a function of time. The performance of these systems is also discussed.