The returned signal has the physical meaning as the loop transmission in the ideal feedback mode. It plays an important role in the study of amplifier stability, its sensitivity to the variations of the parameters, and the determination of its transfer and driving point impedances. This chapter aims to study amplifiers that contain a multiplicity of inputs, outputs, and feedback loops. They are referred to as the multiple-loop feedback amplifiers. It examines the concept of return difference for a controlled source to the notion of return difference matrix for a multiplicity of controlled sources. The chapter explores the null return difference matrix and discusses its physical significance. It demonstrates that the determinant of the overall transfer function matrix can be expressed explicitly in terms of the determinants of the return difference and the null return difference matrices, thereby allowing to generalize Blackman’s formula for the input impedance.