Bioavailabilty is context-dependent. For contaminants in soil, the critical conditions include the nature of the contaminant-matrix and organism-matrix interactions. This lecture will discuss the degree of understanding of the effects of the former on bioavailability. Contaminant molecules must pass from the matrix components (soil particles, soil solution and soil gas phases) through the critical membrane that governs molecular transport into the receptor, for example, the cell membrane in the case of in situ microbial degradation or the epithelial lining in the case of particle ingestion by animals. Contaminant molecules distribute among the soil, liquid, gaseous, and critical receptor membrane phases of the system, and bio-uptake—hence, toxic effect—is governed by thermodynamic and kinetic laws that control this distribution. Molecules in the soil-bound state appear to be much less bioavailable than fluid-phase molecules; hence, bio-uptake is critically dependent on the sorption behavior. Sorption is affected by concentration, presence of competing molecules, hysteresis, and rates.