This Topic formally introduces the abstract definitions for energy and power for measuring the size of a signal. Energy is the measure or metric for non-periodic signals that decay away in the limit. Several examples of calculating energy are worked through, including what happens if you try and calculate energy for signals such as the step signal. This leads to the reason that power signals are defined, and this is the measure for signals that do not decay away, or are periodic. A derivation of the power formula is provided, so that the origin of the calculation is not a mystery. While energy and power are abstract concepts in the context of measuring the size of measuring an abstract signal, they can be converted to real units by considering the dimensions of the signals involved. Finally, there is a simplified expression for the power of a signal, and this is derived at the end of this Topic.