In the drive toward achieving controlled thermonuclear fusion using magnetic confinement of high temperature plasmas, there is a critical need for detailed knowledge of essential plasma parameters such as electron density and temperature. Lasers have made significant contributions to the magnetic confinement fusion program by providing nonperturbing measurements of almost all desired parameters with high spatial and temporal resolution. A wide variety of lasers have been utilized with wavelengths covering the range from the vacuum ultraviolet (-1200 A) to millimeter regions of the spectrum. To illustrate the impact that lasers have made in plasma diagnostics, this paper reviews the selected techniques of laser interferometry, Thomson scattering, and laser resonance fluorescence, together with the relevant laser require-ments. The current state of the art is described, and future needs are projected. The successes already achieved will ensure an expanding role for lasers in the fusion energy program.