This paper presents the theoretical basis of eddy current testing. Firstly, it introduces the impedance analysis method used in the field of eddy current testing. Then we analyze the skin effect and the phase lag on that basis. Next we introduce the field analysis method in eddy current testing. This article takes the Maxwell’s equations of time-harmonic electromagnetic field as a starting point and then describes both the equations and their boundary value problems with vector magnetic potential, and it studies the perturbed magnetic field caused by ideal defects on that basis. Finally, this paper analyzes and contrasts the impedance analysis method and the field analysis method in depth.