Quantitative Structure-Biodegradability Relationships (QSBRs), relate the molecular structure of an organic chemical to its biological degradability. The high complexity of the microbial degradation process as well as great variety of the interactions between organism, organic pollutant and the environment make it difficult to interpret the results from biodegradability experiments. Literature concerning the various approaches applied in Structure-Biodegradability Relationship modeling is reviewed and the reliability and applicability of the results obtained from different biodegradation tests is discussed. A mechanistic approach, based on comparison of the data measured at different organization levels, will hopefully contribute to a better understanding of mechanisms taking place during the biodegradation. Their description and quantification may lead to improvement of existing QSBR models.