A process for forming a three-dimensional article in sequential layers in accordance with a digital model of the article. The process comprises the steps of defining a layer of powder or liquid material, applying a liquid reagent to the layer in a pattern corresponding to the digital model, and repeating these steps to form successive layers. The powder or liquid material comprises a first reactive component and the applied liquid includes a second active component capable of reacting with the first reactive component so that the article is built up in layers. The characteristics of the layers vary across those layers and/or the properties vary from layer to layer.