To form a spatial pattern requires positional information. This information is carried by gradients of signaling molecules (usually proteins) called morphogens. How are morphogen gradients formed? In the simplest case, the morphogen is produced at a certain source position and diἀuses into the region that is to be patterned, called the field. A concentration profile is formed, in which the concentration of the morphogen is high near the source and decays with distance from the source. The cells in the field are initially all identical and can sense the morphogen by means of receptors on the cell surface. Morphogen binds the receptors, which in turn activate signaling pathways in the cell that lead to expression of a set of genes. Which genes are expressed depends on the concentration of morphogen. The fate of a cell therefore depends on the morphogen concentration at the cell’s position.