This chapter discusses factors affecting processes of litter decomposition in coastal wetlands, including biotic, abiotic, as well as anthropogenic factors. A few indicators of litter decomposition are used to describe litter disappearance, including decomposition rate, mass loss percentage, half-life time, and residence time. We examine the variability of leaf litter, wood and root decomposition due to factors such as media for decomposition experiment, stoichiometry, and ecosystem types. Leaf litter decomposes at significantly higher decomposition rate in the aerial (0.06±0.018day−1) than the aquatic environment (0.025±0.007day−1) for mangroves, while nonsignificant differences are found for saltmarshes. There is a significant negative relationship between leaf litter decomposition rate constant and leaf litter C/N ratio for both mangroves and saltmarshes. Both leaf and root decomposition rates differ among wetland geomorphology due to differences in biotic and hydrological conditions. Relatively few studies examine the decomposition of wood compared with leaf litter and roots, but generally mangrove wood is more recalcitrant than leaf tissues. The fate of root carbon in mangroves and saltmarshes is revealed with emitted carbon gases accounting for 27.3% of belowground carbon mineralization for mangroves but unknown for saltmarsh due to the paucity of data. Sediment organic matter in coastal wetlands consists of diversified organic compounds, including carbohydrates, proteins, lipids, and phenols, which are classified into labile or humic substances. Organic matter mineralization involves the breakdown of large to small molecule weight organic compounds and eventually to carbon gases (i.e., CO2 and CH4) via various aerobic and anaerobic pathways. Free energy yields determine the likelihood of the reactions of CH4 and CO2 production. Carbon mineralization is related to a series of processes, including inorganic carbon dynamics and sedimentation in coastal wetlands.