Invertebrates are known to be an appropriate tool for assessment of the state of aquatic ecosystems. Some insect groups are frequently used in alternative test methods due to their specific characteristics such as: higher sensitivity than in vertebrates, short live cycles, large population size, availabity at a very low cost, etc. Species of family Chironomidae are very suitable invertebrates for risk assessment within the scope of programs of aquatic ecosystem and human health protection. They are widely distributed, sensitive to many pollutants; some of them can be cultivated; they have a short lite cycle; and their larvae are of great importance in aquatic food chains. They have excellent salivary gland chromosomes, which are promising tools to assess genotoxicity in the environment. Due to their good resolution they allow detection of a broad range of cytogenetic aberrations at the structural and functional levels. Somatic structural aberrations (inversions, deletions, deficiencies, amplification, and others) can be used as biomarkers at the cytogenetical level for measurement of the toxic effects of different stress agents in the environment. Due to deletions in chromosome G in some chironomid species, it was transformed to so called pompon-like chromosome G, which can be used to test the genotoxicity of various agents in an aquatic ecosystem. It is emphasized that the BR system is a very interesting model for studying the response of the genome to pollutants tn the environment. Together with the standard activity of this system, a clear reversed level of activity was observed. Changes of the activity of the Nucleolar Organizer were also detected. It is shown that these important key structures are affected by trace metal toxicity. The protective role of heterochromatin is discussed. It has been shown that in the model ecotoxicological species C. piger and C. riparius the distribution of somatic rearrangements in their genome is nonrandom. They concentrate mainly at or close to sites rich in repetitive DNA loci. Structural and functional aberrations in polytene chromosomes can serve as early-warning indicators of environmental pollutions. Changes at the population and community levels can be analyzed by using these biomarkers of individual response.