Most biogradable polymers have been designed to have hydrolytically instable bonds in the polymer backbone. By cleavage of these labile bonds, the generally water-insoluble macromolecules are converted into water soluble oligomers. This process may occur in the bulk of the material as well as at the surface depending on the hydrophobicity of the polymeric backbone and stability towards hydrolysis of the functional groups incorporated. Polyester-amides, copolymers of ¿-amino acids and hydroxy acids have been synthesized by the ring-opening copolymerization of cyclo(α-amino acid-α-hydroxy acids) and lactones as lactide and ¿-caprolactone. The in vitro degradation of polyesters and poly(ester-amides) can be described by bulk hydrolysis of ester bonds which results in a decrease of the molecular weight. Weight loss starts when the molecular weight of the polymer chains has sufficiently decreased. The period required for complete degradation depends on the type of polymer, the crystallinity, the initial molecular weight of the samples, and the glass transition temperature. The kinetics of the hydrolysis of ester bonds in polyesters and polyesteramides have been analyzed by determining the molecular weight as a function of time before the start of weight loss. During this period the molecular weight distribution and overall composition does not change. The degradation kinetics of the polyesters and polyesteramides can be described either as a process autocatalyzed by the generated carboxylic acid end groups, or as a non-catalyzed hydrolysis.