This document discusses long-run average costs. It explains that in the long-run, all costs are variable. It describes diminishing returns to increasing quantities of capital and labor. The long-run average total cost curve shows the lowest attainable average cost for different output levels when plant size and labor are varied. Economies of scale occur when a percentage increase in output exceeds a percentage increase in inputs, leading to falling long-run average costs. Diseconomies of scale arise from management complexity at large scales, causing long-run average costs to rise. Constant returns to scale yield a horizontal long-run average cost curve. Minimum efficient scale is the smallest output where long-run average cost is lowest.