This chapter reviews methods and approaches to determine bioavailable metals. It discusses potentials and limitations of existing concepts, presents data from various bioavailability studies on selected metals, focuses on soils, soil-dwelling organisms, and plants and concerns biogeochemistry of selected metals. Soil plays a key role in terrestrial ecosystem functioning. It is also possible to calculate metal speciation in soil solution. Sequential extractions were developed to study metal speciation and behavior in soils and sediments. Regulatory guidelines concerning soil quality criteria or risk assessments often refer to the total metal concentration in soil which is determined by digestion procedures. C. Zhang and W. Davison proposed the so-called diffusive gradients technique in thin films to separate free metals from organically complexed metals in aqueous solutions. A bioassay is a way to test for the presence of a chemical contaminant by using living organisms as an indicator. A number of bioassays have been introduced that measure mortality, growth, reproduction, or behavior of organisms.