This chapter presents a discussion of the basic theory associated with analog filters. The basic elements of circuit theory and the types and applications of analog filters are presented, together with the main features of first-, second-, and higher-order active and passive analog filters. The focus then shifts to a discussion of the design parameters of analog filters and a presentation of the very important Paley–Wiener theorem. The relationship between the amplitude and phase responses of analog filters is analyzed in detail, together with the polynomial and rational approximations of the ideal filter transfer function, such as the Butterworth, Chebyshev I and II, elliptic (or Cauer) filters, and Bessel–Thomson approximations and their main features (e.g., poles and zeros, impulse and frequency responses). The chapter concludes with a discussion of frequency transformations applied to analog filters, which allow for the conversion of a low-pass filter into high-pass, band-pass, and band-stop filters.