This chapter illustrates how Gammarus can be used for assessing the impact of chemical stressors in freshwaters. Organism-level in situ tests can address some of the limitations of community-based biomonitoring methods, whether we understand the implications of changes in the biochemistry or physiology of an organism for the population and community to which it belongs. To this end, we must link effects at different levels of biological organization. We demonstrate how modeling is used to address some of the limitations of the in situ tests, namely: (1) the definition of reference values to address the influence of confounding factors and enable implementation at large spatial and temporal scales and (2) the use of population modeling to improve the ecological relevance of transplantation methodologies for water quality assessment. Finally, we report recent developments in the study of phylogenetic diversity within Gammarus and its implications for their use in assessing water quality.