This chapter describes the conversion process and its limitations and discusses methods to reduce the impact of these limitations in various applications. The process of converting analog signals into digital signals can be divided into two parts. First, the signal will be measured at discrete time instants and then quantized into fixed amplitude levels. The key parameters to consider when selecting an analog-to-digital converter (ADC) or digital-to-analog converter (DAC) are the resolution of the quantizer in number of bits and the sampling rate of the sampler. The chapter discusses these two functions with a basic example. It introduces some commonly used sampling techniques for improving the performance of an existing ADC, such as oversampling, sigma-delta conversion, dithering, time-interleaved sampling, undersampling, harmonic sampling, time-equivalent sampling and post-correction methods. The theory behind signal reconstruction is discussed with respect to the sampling theorem.