This chapter presents the important coupling modes involved in electromagnetic interference (EMI) as well as some EMI regulations. It presents data on average and worst-case electromagnetic emissions found in the environment. The chapter considers the coupling, or crosstalk, to be via mutual inductance and intercircuit capacitance, although one of coupling modes usually predominates. It deals with radiated and conducted components of the electromagnetic environment. The chapter utilises the relatively low-level military broadband radiated emission limit as a reference when comparing natural and manmade emission levels. It describes the term broadband field loosely to indicate a field comprised of a number of frequencies. The subject of broadband and narrowband measurements is dealt with in and sources of broadband noise in. The preceding summary of measured spectrum occupancy and radiated emission levels from unintentional and intentional sources is intended to provide guidelines to manufacturers whose equipment must achieve electromagnetic compatibility in the harsh environments.