The paper introduces the concept of jet areas to quantify a jet's susceptibility to contamination from pileup and underlying event (UE) radiation. Two definitions of jet areas are proposed: passive and active. Passive areas measure a jet's sensitivity to pointlike UE radiation, while active areas measure sensitivity to diffuse UE radiation. The paper investigates these areas for three jet algorithms: $ k_t $, Cambridge/Aachen, and SISCone. For single-particle jets, passive areas are equal to the naive geometrical expectation $ \pi R^2 $, but acquire an anomalous dimension at higher orders in the coupling. Active areas differ significantly, especially for cone algorithms like SISCone, where the jet area is smaller than $ \pi R^2 $. The paper compares these results with direct measures from parton-shower Monte Carlo simulations and finds good agreement with analytical predictions. It also justifies the use of jet areas to subtract pileup contamination. The paper discusses the properties of jet areas for different configurations and shows that the active area is more sensitive to the structure of the event. The results are applied to study the effects of pileup and UE on jet clustering, and the paper concludes that jet areas provide a useful tool for understanding the impact of non-perturbative effects on jet kinematics.The paper introduces the concept of jet areas to quantify a jet's susceptibility to contamination from pileup and underlying event (UE) radiation. Two definitions of jet areas are proposed: passive and active. Passive areas measure a jet's sensitivity to pointlike UE radiation, while active areas measure sensitivity to diffuse UE radiation. The paper investigates these areas for three jet algorithms: $ k_t $, Cambridge/Aachen, and SISCone. For single-particle jets, passive areas are equal to the naive geometrical expectation $ \pi R^2 $, but acquire an anomalous dimension at higher orders in the coupling. Active areas differ significantly, especially for cone algorithms like SISCone, where the jet area is smaller than $ \pi R^2 $. The paper compares these results with direct measures from parton-shower Monte Carlo simulations and finds good agreement with analytical predictions. It also justifies the use of jet areas to subtract pileup contamination. The paper discusses the properties of jet areas for different configurations and shows that the active area is more sensitive to the structure of the event. The results are applied to study the effects of pileup and UE on jet clustering, and the paper concludes that jet areas provide a useful tool for understanding the impact of non-perturbative effects on jet kinematics.