One approach for improving the quality of stormwater runoff before it enters the receiving water is to install stormwater best management practices (BMPs). Stormwater BMPs include a wide range of systems (rain gardens, ponds, wetlands, underground proprietary sediment removal devices, etc.) used to reduce stormwater runoff quantity or improve stormwater quality, or both. No standard methodology for the assessment of stormwater BMPs is available. Therefore, we propose a tiered approach to stormwater BMP assessment that is termed the "Four levels of assessment" (Table 1), that are numbered in order of increasing difficulty and cost. Developers of an assessment program should consider each of the four levels of assessment in order, and consider advancing to the next level only when requirements of the assessment program have not been satisfied. All stormwater BMP assessment programs should include regularly scheduled (at least annual) visual inspections (level 1). In addition to visual inspections, capacity testing (level 2) and/or simulated runoff testing (level 3), if warranted, should be included in stormwater BMP assessment programs at regular intervals to determine the performance of a BMP immediately after installation and to determine how performance of a stormwater BMP is changing with respect to time, changes in the watershed, or both. If the goals of the assessment program cannot be met by capacity testing or simulated runoff testing, or these techniques are not feasible, then monitoring (level 4) should be implemented as part of the assessment program.