Pesticides, in particular herbicides, are regularly measured in Swiss stream water. Therefore, it is important to assess the risk introduced by this pollution to aquatic life. A risk assessment follows three steps: 1. the evaluation of pesticide concentration in surface water, 2. the estimation of a toxicity threshold for these substances, and at the end, 3. the comparison of these values to assess the risk to the environment. The aim of this publication is a critical presentation of the risk assessment method for pesticides. For demonstrative purposes, the method will be applied to three different pesticides (atrazine, metolachlor, diuron) in selected surface waters.