As the foundation of transient electromagnetic field theory, this chapter mainly introduces the basic principles, observation methods and techniques, and inversion methods. The physical basis of electromagnetic detection is the propagation characteristics of the electromagnetic field in the medium, and its mathematical description is the Maxwell’s equations. Based on the Maxwell’s equations, this chapter will sort out the current detection transient electromagnetic methods based on different forms of source devices. This chapter also introduces the propagation characteristics of transient electromagnetic fields from electrical sources.