This chapter discusses the methods for impact assessment of contaminants in sediment. An assessment of the risks of sediment contaminants should include consideration of the bioavailability of the contaminants. Sediment quality criteria are often derived from water quality criteria by multiplication of the latter by sediment to water partition coefficients. Although, this equilibrium-partitioning model has been successful in accounting for the distribution of functionally diverse chemicals in many laboratory-based sorption studies on a wide range of soils and sediments, the ability of this procedure to account for the toxicity of the chemical has been less successful. Despite this latter observation, this simple approach has received broad recognition and formed the basis for several environmental quality standards. These widely employed phase-distribution models based on partitioning with bulk sediment or organic carbon for hydrophobic organic compounds (HOCs) developed in simplified laboratory systems are frequently unable to accurately predict the actual solid-water distribution of chemicals in aquatic field environments.