The chapter starts with a brief description of some of the most important telescopes for radioastronomy, and the distinction between single-dish telescopes and radio interferometers (with special attention on the ALMA array, a very successful facility particularly in the study of protostellar and protoplanetary systems). The chapter then highlights some of the most important discoveries that radio astronomy has contributed. Special attention is given to the discovery of (1) the 21-cm neutral hydrogen line and its importance in the study of the rotation of spiral galaxies; (2) the Cosmic Microwave Background and the birth of modern cosmology; (3) pulsars, with specific discussion of binary pulsars and millisecond pulsars; (4) gravitational lensing, first observed in terms of multiple images of distant quasars, with some description of weak and strong lensing and the interesting measurement of time delays in relation to the determination of the Hubble constant. The discovery of pulsars interpreted as fast rotators prompts a final section, which is a digression on the interplay among density, shape, and rotation in self-gravitating systems.