We start the book with the basics. In this chapter, we first present the motivation and fundamentals of discrete-time (DT) radio-frequency (RF) signal processing, and an overview of zero/low intermediate-frequency (IF) and superheterodyne receiver architec-tures. Then, different sampling schemes present in the state-of-the-art zero-IF DT receivers are studied using a simplified DT receiver. At the end, a 4×-sampling concept is introduced for use in DT high-IF receivers.