This chapter argues that return difference is the difference between the unit applied signal and the returned signal. It explores the amplifiers that contain a multiplicity of inputs, outputs, and feedback loops. They are referred to as the multiple-loop feedback amplifiers. The chapter discusses the concept of return difference for a controlled source to the notion of return difference matrix for a multiplicity of controlled sources. It introduces the null return difference matrix and also discusses its physical significance. The chapter demonstrates that the determinant of the overall transfer function matrix can be expressed explicity in terms of the determinants of the return difference and the null return difference matrices, thereby allowing us to generalize Blackman’s formula for the input impedance. A direct extension of the null return difference for the single-loop feedback amplifier is the null return difference matrix for the multiple-loop feedback networks.