The chapter starts the treatment of convolution codes, first providing the formal description with its generator and parity-check matrices in Toeplitz and “Forney” form, where the former is known as convolution matrix and the latter from the z-domain counterpart in signal processing. The Forney matrix is used to derive a systematic encoder. Just like in signal processing, the canonical structures are presented. As graphical representations, the tree, the state diagram, and the trellis are introduced. The tree is the basis for sequential decoding; the state diagram is used to derive code properties, especially the transfer function with the distance spectrum (weight distribution) to be used in the Union-Bound BER approximation; the trellis as the structure for Viterbi and BCJR decoding. Catastrophic encoding and some aspects of minimality are discussed, too, and the chapter concludes with tables of maximum free-distance convolutional codes.