Law of Diminishing Returns states: As additional units of a variable input are added to fixed inputs, marginal product eventually declines. Explanation: Additional workers on fixed land produce less marginal output because they're less productive (congestion, inadequate tools). Three stages: Stage 1: Increasing returns (MP > AP, AP rises), under-utilized inputs. Stage 2: Diminishing returns (MP < AP, AP falls), optimal stage for production. Stage 3: Negative returns (TP falls, MP negative), too many workers, counterproductive. Optimal production: Stage 2, where AP is maximum. Real example: Adding farm workers on fixed land—initially helps, eventually creates overcrowding, reduces output per worker. Manufacturing: Adding workers to factory with fixed machines—initial efficiency, then congestion, then inefficiency. Why it occurs: Fixed inputs become bottleneck; variable inputs become overused relative to fixed inputs. ICAI questions: Identifying stages, understanding reasons, optimal input levels. Exam tip: "Diminishing returns begin when MP reaches maximum" (not when it becomes zero or negative); Stage 2 is optimal production zone.