This study examines people's comprehension of stock and flow systems, a class of problem that, despite its ubiquity and importance, has been mostly overlooked by management researchers. Stock and flow (SF) systems involve resources that accumulate their inflows less their outflows. Arising in domains from accounting to zoology, examples of stocks include inventory, debt, non-renewable fuels, and pollution. In a series of six experiments we demonstrate that highly educated people have great difficulty understanding even the simplest SF systems. We argue that the difficulty is related to how people represent these systems cognitively, not inability to interpret graphs or text, prior contextual knowledge, motivation, or cognitive capacity. We find that people's inability to properly understand the accumulation or depletion of a stock is a robust error, and that at least part of the difficulty arises from people's inability to properly represent the basic principles governing accumulation. We argue that the SF class of problems may be inherently difficult, much like probabilistic judgments. Given the ubiquity of SF systems, and their importance in making informed judgments, we call for further research to understand how to improve people's intuitive ability to understand processes of accumulation.