For more than 25 years the Standard Model of particle physics has withstood the confrontation with experimental results of increasing precision, but this does not imply that the Standard Model can answer all questions about the ultimate constituents of nature. This book presents a critical examination of the latest experimental results and confronts them with the predictions of the Standard Model. Besides discussions of accelerator results from LEP, HERA and the TEVATRON, attention is paid to the unresolved problems of neutrino oscillations, CP violation, dark matter and cosmology. New theoretical ideas are also analyzed in order to explore possible extensions of the standard model. Realistic plans for future accelerators are presented and their physics potential is discussed, paving the way for the next generation of particle physics experiments.