There is growing recognition that preferential pathways can play an important role at sites affected by vapor intrusion (VI). Although this pathway is often mentioned in regulatory guidance documents, there is little information detailing the conceptual model or prevalence of this pathway. There is also limited guidance on how to assess sites for preferential pathways. As a result, preferential pathways are not currently being investigated in a consistent manner. The goal of this ESTCP project was to obtain a better understanding of sewers and utility tunnels as preferential pathways for VI. Specifically, the project involved developing a conceptual model for this pathway, identifying risk factors, and developing and validating an investigation protocol.