The next major step for the highest possible angular resolution in astronomy will be taken with the construction of large interferometric arrays. ESO's VLT Interferometer will employ an array of four 8 m and three 1.8 m diameter telescopes coherently combined to reach an unprecedented resolution of a millisecond of arc in the visible/IR for objects as faint as 24th visual magnitude. Thus it will open up vast new frontiers in modern astronomy. This book is in preparation for the first observations expected at the beginning of the next century. All areas to be seriously mined with this new facility are presented in a clear and brief exposition with an emphasis on what the VLTI can actually accomplish. Since optical interferometry is still a rather unfamiliar technique for many astronomers, the book contains four short but thorough tutorials that can be used as basic references to the art of interferometry. It will, therefore, also be useful for graduate students.