First-quantized free-particle and random paths, the Nambu-Goto and Polyakov strings, Veneziano amplitudes. The classical bosonic string: old covariant approach, the no-ghost theorem and the existence of a critical dimensionality of space-time, gauge invariances. Light-cone formalism, the Hagedorn temperature. Modern covariant quantization, ghosts, and the BSRT symmetry. Global properties of string theory, multiloop diagrams and the moduli space, strings on curved backgrounds. The fermionic string: classical theory and world-sheet supersymmetry, the GSO projection, spectrum and space-time supersymmetry. Non-Abelian gauge symmetries in open strings. The heterotic string, compactifications on tori. Tree-level amplitudes in the fermionic and heterotic strings.