The iron enhanced sand filter (IESF) is a popular stormwater control measure in Minnesota, employed to reduce phosphorus inputs to water bodies from urban storm runoff. Although IESFs have been employed for over a decade, research and development of technology remains limited. There is a large amount of anecdotal information from practitioners but often this leads to more questions than answers. The Minnesota Stormwater Research Council funded a project to combine the observations and experiences of IESF operators with field monitoring and chemical analysis to expand our fundamental understanding of the relationships between design, maintenance, performance and lifespan of IESFs. We collected water quality data and analyzed media samples from 24 different IESFs throughout the Twin Cities Metro Area. We developed an empirical method for estimating total phosphorus (TP) retention over time and investigated the correlation between TP retention and depth of water treated by the IESF. We also investigated the correlations between soluble reactive phosphorus (SRP), the depth of water treated, the total phosphorus to total iron ratio of the IESF media, and the capacity remaining as determined by batch tests. Our results demonstrate the usefulness of some straight-forward measurable IESF parameters in understanding the performance of an IESF over its lifespan. Coupled with information gathered from the IESF operators who participated in this study, we have expanded the guidelines for proper maintenance to maximize the performance and lifespan of an IESF.