The article discusses the concept of jet areas, which measure the susceptibility of jets to radiation from pileup or underlying events (UE). Two main definitions of jet areas are introduced: passive and active areas. Passive areas, defined using a single infinitely soft particle (a "ghost"), characterize the sensitivity to pointlike UE radiation, while active areas, defined using a dense coverage of ghosts, characterize the sensitivity to diffuse UE radiation. The authors investigate these areas for three standard jet algorithms—$k_t$, Cambridge/Aachen, and SISCone—and find that passive areas for single-particle jets are equal to the geometrical expectation $\pi R^2$, but acquire an anomalous dimension at higher orders in the coupling. Active areas differ from $\pi R^2$ even for single-particle jets, especially for cone algorithms like SISCone with a Tevatron Run-II split-merge procedure. The authors compare their results with Monte Carlo simulations and find good agreement. They also justify the use of jet areas to subtract contamination from pileup. The article covers the properties of passive and active areas, area scaling violations, and the impact of UE/PU on jet clustering.The article discusses the concept of jet areas, which measure the susceptibility of jets to radiation from pileup or underlying events (UE). Two main definitions of jet areas are introduced: passive and active areas. Passive areas, defined using a single infinitely soft particle (a "ghost"), characterize the sensitivity to pointlike UE radiation, while active areas, defined using a dense coverage of ghosts, characterize the sensitivity to diffuse UE radiation. The authors investigate these areas for three standard jet algorithms—$k_t$, Cambridge/Aachen, and SISCone—and find that passive areas for single-particle jets are equal to the geometrical expectation $\pi R^2$, but acquire an anomalous dimension at higher orders in the coupling. Active areas differ from $\pi R^2$ even for single-particle jets, especially for cone algorithms like SISCone with a Tevatron Run-II split-merge procedure. The authors compare their results with Monte Carlo simulations and find good agreement. They also justify the use of jet areas to subtract contamination from pileup. The article covers the properties of passive and active areas, area scaling violations, and the impact of UE/PU on jet clustering.