Most people know that electrically active cells in nerves, in the heart and in the brain generate electrical currents, and that somehow these result in measurements we all have heard about, such as the electrocardiogram. But how? That is, what is it that happens within the electrically active tissue that leads to the creation of currents and voltages in their surroundings that reflect the excitation sequences timing, and condition of the underlying tissue. This course explores that topic. Rather than being a primer on how to interpret waveforms of any kind in terms of normality or disease, the goal here is to provide insight into how the mechanism of origin actually works, and to do so with simple examples that are readily pictured with simple sketches and one’s imagination, and then moving forward into comparison with experiments and finding outcomes quantitatively.