The law of diminishing marginal utility states that as consumption of a good increases, the additional utility from each additional unit decreases. It assumes utility is measurable, consumers aim to maximize utility, units are homogeneous and consumed continuously. Marginal utility decreases as total utility increases, as shown in an example table and graph. Applications include explaining downward sloping demand curves and the concepts of consumer surplus and progressive taxation. Limitations are if units are not homogeneous, consumption is not continuous, or income or prices change.