This Topic starts this Chapter on the theoretical glue that is needed to fully understand Nyquist sampling theory, that follows in the next Chapter. Sampling theory requires the understanding of the Dirac delta impulse function. This Topic introduces this impulse function by first considering the unit step function. The application of the step signal for representing causal functions is first presented. The gradient of the step function is then considered, and used to motivate the development of the impulse function. The relationship between the step and impulse functions is explained.