The concept of a reservoir compartment is controversial, and researchers have different opinions about how to define the terms compartment and compartment boundary and about what reservoir properties should be used to support those definitions. A popular definition is that two points (A and B) within a reservoir system are in different compartments if oil or gas cannot flow between points A and B in a time period that satisfies the economic requirements for production. This definition is popular with people who judge reservoir connectivity from the economic view of how efficiently fluid moves within the reservoir system. Other people think that reservoir compartments should be defined on the basis of some fundamental variation in the petrophysical properties of the reservoir facies, such as a significant change in porosity or permeability. A third community of people prefers that observed differences in engineering-related parameters, such as reservoir pressure, production rate, or fluid chemistry, be used to define the internal compartmented architecture of a reservoir system. In this module, we will infer the presence or absence of compartment boundaries by observing effects on pressure pulses that travel through the pore spaces that connect perforation zones in our well with perforation zones in another well.