Establishing bias currents and voltages for building blocks comprising an overall design is fundamental to the design of bipolar integrated circuits. These building blocks include single-stage and differential amplifiers, output stages, Biasing often has a direct relationship to electrical characteristics, such as gain, signal-swing, slew-rate, of the individual building blocks and hence to the overall design. Biasing circuits include current sources, voltage references, and level-shifters. Efforts to improve the performance of current sources and voltage references have led to many refinements and developments that have started from simple beginnings. The chapter presents some of the fundamental current sources and voltage references used for biasing in bipolar integrated circuit technologies and refinements of these circuits that have evolved over time. The current mirror is a circuit that reproduces a reference current at one or more locations in larger circuit. Parameters and component values for fabricated devices usually exhibit deviation from some “nominal” value during the fabrication process.