This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book describes the nature of solar radiation and provides the terminology and basic equations used in solar and infrared radiation measurements. It presents broadband solar radiation measurements, instrumentation, estimating measurement uncertainty, and radiometer calibrations. The book discusses instruments used to measure direct normal irradiance or “beam” irradiance and the instruments used to measure the total irradiance on a horizontal surface. It addresses the more challenging aspects of measuring and modeling the spectral distributions of solar radiation. The book provides an overview of measuring the surface albedo, which illustrates some of the complexities of how the Earth’s surface reacts to radiation from the sun and sky and the calibration of radiometers for measuring broadband and spectral irradiance. It also discusses best practices for the operation and maintenance of solar monitoring stations.