Apparatus for generating a three-dimensional object, comprising: a build material distributor to provide a first layer of build material on a build material support, and to provide subsequent layers of build material on a previously provided layer of build material; a first agent distributor including a supply of coalescing agent and configured to selectively deliver the coalescing agent onto portions of a layer of build material; a second agent distributor including a supply of coalescence modifier agent and configured to selectively deliver the coalescence modifier agent onto portions of a layer of build material; an energy source to apply energy to build material; and a controller to: control the build material distributor to provide a layer of build material; 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, so that when energy is applied to the layer the build material coalesces and then solidifies to form a slice of the three-dimensional object in accordance with the patterns, such that portions of the layer on which only coalescing agent is delivered or penetrates heat up above the melting point of the build material and coalesce, portions of the layer on which only coalescence modifier agent is delivered or penetrates do not to heat up above the melting point of the build material and not to coalesce, and portions of the layer on which coalescing agent and coalescence modifier agent are delivered or penetrate undergo a modified degree of coalescence; and control the energy source to apply energy to the layer of build material. a build material distributor to provide a first layer of build material on a build material support, and to provide subsequent layers of build material on a previously provided layer of build material; a first agent distributor including a supply of coalescing agent and configured to selectively deliver the coalescing agent onto portions of a layer of build material; a second agent distributor including a supply of coalescence modifier agent and configured to selectively deliver the coalescence modifier agent onto portions of a layer of build material; an energy source to apply energy to build material; and a controller to: control the build material distributor to provide a layer of build material; 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, so that when energy is applied to the layer the build material coalesces and then solidifies to form a slice of the three-dimensional object in accordance with the patterns, such that portions of the layer on which only coalescing agent is delivered or penetrates heat up above the melting point of the build material and coalesce, portions of the layer on which only coalescence modifier agent is delivered or penetrates do not to heat up above the melting point of the build material and not to coalesce, and portions of the layer on which coalescing agent and coalescence modifier agent are delivered or penetrate undergo a modified degree of coalescence; and control the energy source to apply energy to the layer of build material. control the build material distributor to provide a layer of build material; 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, so that when energy is applied to the layer the build material coalesces and then solidifies to form a slice of the three-dimensional object in accordance with the patterns, such that portions of the layer on which only coalescing agent is delivered or penetrates heat up above the melting point of the build material and coalesce, portions of the layer on which only coalescence modifier agent is delivered or penetrates do not to heat up above the melting point of the build material and not to coalesce, and portions of the layer on which coalescing agent and coalescence modifier agent are delivered or penetrate undergo a modified degree of coalescence; and control the energy source to apply energy to the layer of build material.