A major challenge to developing mechanistic models of soil organic matter dynamics is linking the activities of microorganisms responsible for decay to changes in substrate mass and chemistry. Another complication is the high diversity of the soil microbial community and our limited ability to evaluate its structure and function. In theory, different microorganisms have different capacities to degrade particular substrates, as well as respond differently to environmental conditions and resource availabilities. In practice, gross indices of temperature and moisture, in concert with substrate lignin and nitrogen contents can be used to predict litter decay with remarkable accuracy, while providing remarkably little insight to the actual process of decay, microbial activity or community dynamics. However, advances in experimental methods are demonstrating that changes of community composition during succession on decaying litter correspond to changes in litter chemistry, and changing activity profiles of extracellular enzymes, suggesting cause-and-effect relationships that may be used to devise more mechanistic models of decomposition. At present, modeling efforts are beginning to link the activities of key groups of enzymes to the degradation of major classes of litter constituents and speculate that these relationships are mediated by particular guilds of microorganisms that vary in community dominance during succession. These models need both structure and mechanism that corresponds to readily measurable characteristics of the microbial community, substrate chemistry and key interactions between microorganisms and substrate (i.e., decay processes).