The invention relates to an injection device comprising an elongate housing and a cap removably mounted over a forward end of the housing. The cap comprising at least one slot extending forwardly from its rear edge the at least one slot being configured to engage with an outwardly projecting protrusion on an outer surface of the housing when the cap is mounted on the housing. A rearward portion of the cap includes rearwardly extending engagement features provided on each side of the at least one slot, the engagement features being configured to locate within a cavity in the housing when the cap is mounted on the housing. The cavity is provided with corresponding surfaces which align with the cap engagement features so as to hold the outwardly projecting protrusion within the cut-out.