The “Galileo affair” is undoubtedly the most often-cited incident in the history of science-religion interactions. Far from being a simple case of science versus religion, however, it is extremely complex and brings up a host of important philosophical, scientific, and other issues that must be understood in context. In this lecture, we look at the background to these events in Copernicus’s heliocentric theory, then detail the intricate events of 1613−1616 - “Act I” of the Galileo affair - and their implications and possible explanations.