**1. Introduction**

The state of the environment is signaled by a group of different biological species known as bioindicators. They are responsible for demonstrating the impact of different types of pollutants. The selection of a specific biological species as an indicator should be done considering its sensitivity to various changes in the environment [1]. Fishes are valuable species as bioindicators for the pollution of water bodies. A very large number and essentially different methods can be applied to them, which allow assessment of the severity of toxic effects by determining the accumulation of toxic substances in the tissues, by using histological and hematological approaches. Thus, bioindication using fish represents a good tool for biomonitoring, regarding both pollution and river engineering aspects, e.g., river restoration and management [2].

In this chapter, our and other authors' summarized data on the possibilities of using the erythrocyte indicators of teleost fishes for the purposes of biomonitoring will be presented.
