Today, probabilistic risk assessments (PRAs) at multi-unit nuclear power plants consider risk from each unit separately and do not formally consider interactions between the units. These interactions make the operation of multiple units dependent on each other and should be accounted for in the PRAs. In order to effectively account for these risks in a multi-unit PRA, six main dependence classifications have been created: initiating events, shared connections, identical components, proximity dependencies, human dependencies, and organizational dependencies. This thesis discusses these six classifications that could create dependence between multiple units. As a validation of the classification, this thesis will also discuss multi-unit events that have occurred in operating plants. Finally, this thesis will present existing methodologies that could be used to quantify unit-to-unit dependencies in the PRA for each classification.