According to one aspect there is provided apparatus for generating a three-dimensional object. The apparatus comprises a first agent distributor to selectively deliver a coalescing agent onto portions of a layer of build material, a second agent distributor to selectively deliver a coalescence modifier agent onto portions of a layer of build material, and a controller to control the agent distributors to selectively deliver each of the agents onto a layer of build material in respective patterns derived from data representing a slice of a three-dimensional object to be generated, wherein the coalescence modifier agent is delivered adjacent to where the coalescing agent is delivered to help reduce the effect of lateral coalescence bleed, so that when energy is applied to the layer the build material coalesces and solidifies to form a slice of the three-dimensional object in accordance the patterns.