Medium-chain-length polyhydroxyalkanoates (mcl-PHAs) are intracellular biopolyesters that function as carbon and energy reserves while enabling elastomeric, biocompatible materials. This chapter summarizes core biology and engineering of mcl-PHAs with emphasis on Pseudomonas putida: precursor supply from de novo fatty acid synthesis and β-oxidation; polymer assembly by class II PHA synthases; and granule organization by phasin proteins that influence carbon allocation and inclusion inheritance. On the catabolic side, intracellular and extracellular depolymerases (α/β-hydrolases) operate at polymer interfaces, where adsorption and polymer microstructure govern rates of hydrolysis. Material properties can be further tailored biologically by feedstock programming, pathway and strain engineering, and cultivation strategies that modulate monomer composition, molecular weight, and inclusion architecture, and chemically through post-polymerization modifications such as epoxidation or grafting. Within a circular-economy framework, mcl-PHAs enable multiple end-of-life options, including mechanical and chemical recycling and managed biodegradation (by industrial composting or anaerobic digestion); however, enzyme-mediated depolymerization stands out as a high-value route because it proceeds under mild aqueous conditions, exhibits high selectivity, and can recover enantiopure (R)-3-hydroxyalkanoates for genuine closed-loop re-synthesis with minimal side-product formation. Collectively, these advances position mcl-PHAs as leading candidates for truly circular, scalable materials, where rigorous cell biology translates into manufacturable products with high performance and low environmental burden.