Abstract: With the transition of wastewater treatment from conventional pollutant removal toward integrated pollution reduction， carbon mitigation， and resource recovery， the valorization of organic components in sewage sludge has attracted increasing attention. Sludge proteins are extensively embedded within extracellular polymeric substances （EPS） and microbial cells and constrained by hydrogen bonding， hydrophobic interactions， electrostatic interactions， metal-ion bridging， and humic-protein associations. These structural constraints result in strong dependence of protein release and subsequent transformation on sludge matrix characteristics. Existing studies have primarily focused on protein solubilization and recovery efficiency， whereas the hydrolysis degree， molecular-weight distribution， and formation of target peptides after protein release remain insufficiently understood. The spatial distribution and structural constraints of proteins in sludge EPS are first summarized， followed by a comparison of thermal， mechanical， chemical， and biological approaches for EPS disruption， protein migration， and hydrolysis. The selectivity of hydrolysis for heterogeneous sludge protein substrates， the product differences associated with endoproteolytic， exoproteolytic and non-enzymatic hydrolysis， and the effects of enzyme type， pH， temperature， reaction time and hydrolysis intensity on the accumulation of peptide products are further discussed. In addition， the evidence for protein-to-peptide conversion is evaluated at multiple levels， including colorimetric analysis， apparent degree of hydrolysis， molecular-weight fractionation， peptide-bound amino acids， native peptidomics and targeted mass spectrometry. Existing studies mainly demonstrate protein release， peptide-bond cleavage and the formation of low-molecular-weight components， whereas sequence-level identification and targeted quantification of native peptides remain limited. Finally， the direct applications of sludge peptide-enriched hydrolysates or low-molecular-weight peptide-enriched fractions in foaming materials， bio-based adhesives and sludge drying enhancement are summarized. A use-oriented framework integrating limited hydrolysis， multilevel characterization and product fractionation is proposed. Future research should combine metaproteomics， time-resolved peptidomics， in silico digestion and targeted quantification to establish a full-process evaluation system linking EPS structural regulation， peptide quality and end-use performance.