Commons problems arise whenever self-interested agents fail to take account of the combined negative spillovers their actions inflict on others. Sometimes these spillovers have only short-run consequences---as when a person enters a packed elevator disregarding everyone else's combined distress or enters a clogged expressway, disregarding the increase in everyone else's combined commute time. Sometimes these spillovers take years to manifest themselves---until the depleted fisheries, drained aquifers, changed climate, and antibiotic- resistant bacteria make their cumulative effects undeniable. This lecture reviews the theoretical literature on commons problems with an emphasis on their occurrence in the context of natural resources. It concludes with analyses of two real-world institutions to limit excessive use of the commons: (1) catch-sharing in Japanese fisheries and (2) prorationing on Texas oil fields.