Within the frame work of a world-wide collabora-tion, various possible approaches for Linear Colliders in the TeV energy range (TLC) and high luminosity (~ 1034 cm-2 sec-1) are explored in different laboratories and periodically compared in international workshops. The main accelerator physics issues required to meet the requested performance improvement by three orders of magnitude in luminosity and by a factor 10 in beam energy with respect to the unique linear collider presently operational, the SLC at SLAC, are reviewed, pointing out the main challenges common to all designs as well as the possible technological choices. Corresponding designs based on the improve-ment of present standard or the development of new tech-nologies are presented, emphasizing their main issues and specific challenges. The main goals of ambitious test facilities presently set-up to study the feasibility and cost of the various schemes in the next few years are introduced
