Petroleum polycyclic aromatic hydrocarbons (PAHs) are hazardous environmental pollutants, and PAH exposure causes public health risks and raises environmental concerns. While it has been demonstrated that PAHs could be degraded by many bacterial isolates, little is known about PAH degradation by yeast, especially for high molecular weight (HMW) PAHs. In this study, the performance of isolated yeast strains to degrade HMW-PAHs was investigated in liquid culture, slurry reactor and different biological wastewater treatment systems. The meta- genomics techniques such as fluorescent in situ hybridization (FISH), 16S r RNA gene clone libraries and denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplified 16S and 26S rRNA genes were used to study microbial structure dynamics. Variations of some functional genes (nahAc & C230) responsible for the degradation of aromatic compounds were also followed. The book is important for both students and researchers wishing to increase their understanding in the field of petroleum biodegradation and environmental meta-genome biotechnology.