Interest in the biodegradation mechanisms and environmental fate of polycyclic aromatic hydrocarbons (PAHs) is motivated by their ubiquitous distribution, their low bioavailability and high persistence in soil, and their potentially deleterious effect on human health. Due to high hydrophobicity and solid-water distribution ratios, PAHs tend to interact with non-aqueous phases and soil organic matter and, as a consequence, become potentially unavailable for microbial degradation since bacteria are known to degrade chemicals only when they are dissolved in water. As the aqueous solubility of PAHs decreases almost logarithmically with increasing molecular mass, high-molecular weight PAHs ranging in size from five to seven rings are of special environmental concern. Whereas several reviews have focussed on metabolic and ecological aspects of PAH degradation, this review discusses the microbial PAH-degradation with special emphasis on both biological and physico-chemical factors influencing the biodegradation of poorly available PAHs. (C) 2004 Elsevier Ltd. All rights reserved.