Work in progress at the U.S. EPA's National Risk Management Research Laboratory is developing methods for quickly, easily, and inexpensively developing Life Cycle Assessment (LCA) inventories. An LCA inventory represents the inputs and outputs from processes, including fuel and raw material feeds and environmental releases. The collection of such inventories can be a time consuming and expensive task. Through an analysis of these flows and their effects, this paper will discuss heuristics for applying shortcut methods. Questions to be answered include: Can case study results using shortcuts be generalized? When is it (not) acceptable to use rough estimates of feeds or releases? Are only certain species bound by these rules? Results of recent studies will provide insight into answering these questions.