An external infusion device for infusing a fluid into a body comprises a first module (200), said module being compact so that it can be worn under clothing. The first module includes a first housing (224), an alarm (222) in the first housing (224), a reservoir in the first housing (224) to hold a fluid, and a pump (210) in the first housing, adapted to pump the fluid from the reservoir. The device also has a second module which includes a second housing, and a display (101, 301) in the second housing. In operation the second module (100) communicates with the first module (200) to provide a fluid delivery program for the flow of the fluid from the first housing (224). The second module (100) being adapted to be attachable to and removable from the first module (200) without interrupting or discontinuing the fluid delivery program thereby enabling the first module (200) to be worn without the second module (100) making the infusion device lighter and less bulky than with both modules. In one illustrated arrangement the first and second modules (200, 100) have sliding or snapping parts (102, 202) that mate to removably attach the first module (200) to the second module (100).
