This chapter explores the concept of frequency response in discrete-time systems, emphasizing its role as a bridge between time-domain filtering and frequency-domain interpretation. The frequency response is introduced as the discrete-time Fourier transform (DTFT) of a system’s impulse response, highlighting how it characterizes the system’s action on sinusoids. We connect this perspective to the z-transform evaluated on the unit circle, illustrating periodicity and symmetry properties. Through simple but instructive examples—low-pass and high-pass finite impulse response (FIR) filters—we show how moving averages and differences translate into distinct spectral behaviors. Python visualizations of magnitude and phase responses illustrate practical computation, interpretation in decibels (dB), and the significance of linear phase. These concepts form a foundation for systematic filter design in the following chapters.