When the weather is right, you might see a brightly colored balloon floating far above you. The technology that goes into a hot air balloon is one that we may not pay much attention to these days. It works by trapping hot gases in a container that remains upright over time. It is the oldest human-carrying flight technology and is still used today for many important applications, such as weather monitoring. In this chapter, we are going to study the properties of gases and the effects that altering one of these properties has on various others.