This chapter discusses several time-domain representations for digital filters, including the difference equation, system diagram, and impulse response. Additionally, the convolution representation for LTI filters is derived, and the special case of FIR filters is considered. The transient response, steady-state response, and decay response are examined for FIR and IIR digital filters, and various matrix formulations of digital filters are discussed, including the state space formulation. In addition, elementary system identification is introduced.