This peer-reviewed article from BioScience is about using mechanistic models to scale ecological processes. Human activities affect the natural environment at local to global scales. To understand these effects, knowledge derived from short-term studies on small plots needs to be projected to much broader spatial and temporal scales. One way to project short-term, plot-scale knowledge to broader scales is to embed that knowledge in a mechanistic model of the ecosystem. The National Science Foundation’s Long Term Ecological Research (LTER) Network makes two vital contributions to this type of modeling effort: (1) a commitment to multidisciplinary research at individual sites, which results in a broad range of mutually consistent data, and (2) long-term data sets essential for estimating rate constants for slow ecosystem processes that dominate long-term ecosystem dynamics. In this article, we present four examples of how a mechanistic approach to modeling ecological processes can be used to make projections to broader scales. The models are all applied to sites in the LTER Network.