Numerous methods exist for quantifying base infiltration (BI) in municipal sanitary sewer systems. York Region staff chose to perform base infiltration analysis using the Stevens-Schutzbach method and the Potable Water Use method. This was possible due to the availability of long-term sewer flow and rainfall monitoring datasets collected as part of the Region’s Inflow and Infiltration Reduction Program and water consumption records collected in over 250 individually metered sewer catchments. The Steven-Schutzbach method is an empirical equation that relates the sanitary average daily dry weather flow to minimum daily dry weather sanitary flow to calculate the base infiltration. The Potable Water Use method compares water consumption data to the sanitary average daily dry weather flow to calculate base infiltration. Results indicate that on average the base infiltration in York Region is approximately 30% of the average daily dry weather flow, with a weighted standard deviation of approximately 15%. Relationships between the physical characteristics of sewer catchments and the quantity of BI were also investigated. Physical characteristics include sewer pipe age, sewer pipe material and sewer flow temperature. The results indicate a weak correlation between the quantity of base infiltration and both pipe age and sewer pipe material. However, a moderate correlation was observed between the quantity of base infiltration and sewer flow temperature. Areas that have a high percentage of foundation drain collectors connected to the sanitary sewer system are also suspected of increasing the base infiltration, however, the flow monitoring data was not available in areas small enough to investigate the potential effect.