Requirements for very high energy accelerators

In this introductory paper at the second Workshop on Laser Acceleration my main goal is to set what I believe to be the energy and luminosity requirements of the machines of the future. These specifications are independent of the technique of accelerations. But, before getting to these technical questions, I will briefly review where we are in particle physics, for it is the large number of unanswered questions in physics that motivates the search for effective accelerators