For over four decades, individuals from various disciplines have extensively researched the tasks comprising manual materials handling (MMH): lifting, lowering, pushing, pulling, carrying and holding. The primary motivation for these research efforts has been a desire to understand human capabilities so that tasks can be designed such that the demands of the task are at or below the capacities of the workers performing the task. Various criteria for defining acceptable task demands have been developed from the principles of biomechanics, physiology, and psychophysics. Although significant bodies of literature exist on each class of criteria, there are still areas that need to be examined. Additionally, the validity of several of the criteria is unknown, primarily due to a lack of epidemiological verification of the criteria. This paper presents a critical review of MMH criteria, the shortcomings of and conflicts between the various criteria, and the areas needing further examination. The critical need for epidemiological studies is also detailed.