A tool (30) for graphically defining an expression is disclosed. The tool comprises a graphic user interface (GUI) component (31) and an expression generator component (32) adapted to read a graphic definition of an expression provided by a user through the GUI component. The GUI component is responsive to user input to define one or more tree structures comprising a hierarchical series of nodes, and one or more lists comprising a plurality of items, each list item being associated with respective node of an associated tree. The expression generator component analyzes the graphic definition and generates an expression based on the structure of the or each tree and any list items associated with respective nodes of a tree.