This chapter reviews a wide range of approximation techniques and circuit design styles, for both discrete and monolithic circuits. It examines discrete-time circuits realized using analog dynamic techniques based on charge transfer, which is very significant for mixedsignal processing and computational microelectronic systems. The chapter explores approximation issues and outlines different techniques for uni and multidimensional functions, emphasizing hardware-oriented approaches. These techniques involve several nonlinear operators and the linear operations of scaling and aggregation. The chapter presents circuits for piece wise linear and piecewise-polynomial functions, describes neural and fuzzy approximation techniques, and provides an extension to dynamic circuits. An extended technique to design nonlinear electronic hardware for both discrete and monolithic design styles uses polynomial approximating functions. A drawback of polynomial and rational approximants is that their behavior in a small region determines their behavior in the whole region of interest.