Today's television installations generally contain a combination of analog and digital, component and composite equipment linked by routers, encoders, decoders and converters. The configuration may be redundant to allow continued use in the case of equipment failures, or expensive, special purpose devices may be shared between studios or edit suites. It will be seen here that the reconfiguration made possible by the use of routers places heavy demands on signal standardization. It does not matter whether the system is analog or digital - correct operation can only be ensured if all devices in a signal chain are designed and adjusted so that the meaning of the signal does not change with the configuration. Analog signals have standardized voltages and timings; digital signals have standardized code values and protocols. This chapter outlines the principles and practice of analog and digital video interfacing. To ensure that standards are met, various tests have been developed. These are described here along with the equipment and techniques needed to make them. The testing procedures for analog and digital equipment are quite different. However, in order to set up an ADC or a DAC correctly, both types of test may be needed together.