Publisher Summary This chapter discusses the basics of wireless communications, such as signal multiplexing, filtering, and modulation. The sharing of a communications channel by multiple users is known as multiplexing. There are a number of methods to successfully locate and receive particular signals and reject others, including frequency-division multiplexing, time-division multiplexing, spatial multiplexing, code-division multiplexing, and directional multiplexing. Any signal that is periodic in time can be regarded as being composed of sinusoids of differing frequencies or described either as a function of time or as a function of frequency by taking its Fourier transform. The orthogonality of differing frequencies makes it possible to extract a signal from a complex mixture of signals of different frequencies, which is generally known as filtering. To transmit information, waves must be modulated, resulting in a finite bandwidth for a given data rate and thus a finite amount of spectrum to share between differing uses and users. The chapter examines various modulation schemes including binary phase-shift keying (BPSK), quadrature-amplitude modulation (QAM), on-off keying (OOK), and amplitude-shift keying (ASK). The chapter also examines two specialized modulation techniques, orthogonal frequency-division multiplexing (OFDM), and pulse radio, of increasing interest in the modern wireless world.