The underlying concept of the bioindicators approach in biomonitoring is that contaminant effects occur at the lower levels of biological organization (i.e., at the genetic, cell, and tissue level) before more severe disturbances are manifested at the population or ecosystem level (Chapter 1, Figure 1). Effects of pollutant stress are usually expressed first at the molecular/biochemical level (where pollutants affect the normal functioning of chemical processes in the body) (Chapter 1, Figure 1). These effects can be detected as changes in enzyme levels, cell membranes, or genetic material (DNA). Changes at these subcellular levels induce a series of structural and functional responses at the next higher level of biological organization, which can impair, for example, complex processes such as hormonal regulation, metabolism, electrolyte balance, and proper functioning of the immune system. These effects, in turn, may eventually alter the organism’s ability to grow, reproduce, or survive. Ultimately, irreversible and detrimental effects may be observed at the population, community, or ecosystem levels of biological organization.