Approximately 23 percent of the estimated 115 million occupied homes in the United States are served by onsite wastewater systems. The vast majority of onsite wastewater treatment systems include a septic tank, grease trap, or both for primary treatment. These units are efficient, simple, low-energy treatment units whose performance is critically important to the overall functioning of onsite wastewater systems. Regulations, industry standards, guidance materials and engineering texts vary widely and are often incomplete in their consideration of the factors that may influence primary unit performance in onsite systems. The objective of this research was therefore to identify, compile, analyze, and report on the existing body of work addressing the performance of primary treatment units in onsite wastewater systems and the factors impacting performance. Design, construction/installation, and maintenance issues were considered, with a goal of establishing what is known, what is not known and what future research is needed in this area. Over 700 sources of information were collected, with most reviewed and presented in this document the white paper. A bibliographic database, which can continue to be updated into the future, was developed as a companion piece to the white paper, as a tool for researchers and practitioners.