The purpose of this report is to provide a perspective on future accelerator projects, and to identify the R&D activities necessary to prepare for these projects. The report summarizes the conclusions of accelerator studies made during the 2001 Snowmass Summer Study on the Future of Particle Physics. In doing so, it serves as a summary of the opinions on accelerator R&D expressed by the scientific community as it looks towards the next few decades. The main technical content is provided by the Executive Summaries of each of the fifteen accelerator Working Groups. These Working Group Executive Summaries form an integral part of this report.