This book presents a systematic and detailed account of the classical and quantum theory of the relativistic string and some of its modifications. Main attention is paid to the first-quantized string theory with possible applications to the string models of hadrons as well as to the superstring approach to unifications of all the fundamental interactions in the elementary particle physics and to the "cosmic" strings. Some new aspects are provided such as the consideration of the string in an external electromagnetic field and in the space-time of constant curvature (the de Sitter universe), the relativistic string loaded by point-like masses and the Cartan method for describing the classical string dynamics. The relativistic membranes and p-branes are also considered briefly. The book is sufficiently self-contained and can be considered as an introduction to this new and fast developing branch of the elementary particle physics.