Water quality treatment systems, such as vegetated agricultural ditches, retention ponds, and water quality treatment wetlands, are used to reduce waterborne contaminant loads. These systems have been gaining prominence for their successful mitigation of pesticides, agricultural ef?uence, and pharmaceutical and personal care products. Aquatic organisms exposed to the out?ow runoff are the most vulnerable to toxicant exposure. While there is clear evidence of successful contaminant remediation within these systems, the link between pesticide concentration reduction and biological responses of aquatic organisms exposed to treated runoff is less clear. Generally, toxicity is reduced as water passes through the treatment systems with accelerated amelioration in those systems where vegetation is present. Contaminant concentrations are often reduced to levels below analytical detection limits, and, in most cases, organism responses (e.g., mortality, growth) are negatively correlated to pesticide concentrations. However, a complete removal of toxicity is rarely observed. Hydrophobic pesticides elicit toxicity in sediment tests, whereas hydrophilic pesticides cause more adverse effects in the water column tests. Pesticides bound to sediment particles were shown to be less bioavailable than those in the water phase. Although there is concern about source/sink dynamics of these treatment ponds in the long term, these treatment processes are an effective way to mitigate pesticide loads in water sources. Studies evaluating treatment system effcacy over time, and whether wetland sediment toxicity increases during that time, are needed to determine the long-term viability of these systems.