The goal of the lesson is to enable a marine forecaster to manually predict how the wave height and period will change as the waves leave their generation area, become swell, and then propagate and disperse into the forecaster’s offshore coastal waters. While numerical wave prediction models can provide swell height and period forecasts, they are dependent on accurate wind forecasts by atmospheric prediction models. Therefore, manual skills in determining swell height and period are needed in order to cross-check or correct model predictions in cases of poor or unresolved model forecasts of winds. The lesson starts by discussing how swell propagate along great circle tracks and how these tracks will look different on various map projections. With this in mind the concept of developing a known “swell window” for a given location is introduced. Next, the lesson uses conceptual animations to demonstrate the effects of dispersion on the swell group as it propagates over a long distance. Also discussed are nonlinear processes, wave steepness, travel time, event duration, and opposing winds. Then the lesson explains how swell height changes due to angular spreading of wave energy and provides a simplified method to calculate this change. Finally, users are able to test their new understanding of these concepts through a short exercise where they are asked to determine swell height and period at multiple locations. User interactions are included throughout the lesson and within the short exercise. This is the third in a series of training lessons on marine wind and waves. It follows the “Wave Types and Characteristics” and “Wave Generation” lessons.