The process of conversion of an analogue signal into a digital version is examined; in particular the errors introduced by the conversion are analysed. The salient features of the main methods of conversion are then described. Since sampling of signals is inherent in the process of conversion, the mathematics of sampled signals is then examined and the discrete forms of the Laplace and Fourier transforms are developed. These are then used to show how signals may be processed in the digital domain. Finally, the principles of the design of circuits for the realisation of digital signal conditioning algorithms are described.