A wound dressing comprising: a therapeutic agent selected from the group consisting of antimicrobial substances, pain relieving substances, protease inhibitors, and mixtures thereof; and a barrier layer for initially separating the therapeutic agent from a wound fluid in use, wherein the barrier layer comprises a substrate for an enzyme selected from the group consisting of proteases, kallikrein and tissue-plasminogen activator. Preferably the substrate comprises a substrate for elastase or a collagenase. The barrier layer breaks down in infected or chronic wounds, thereby releasing the therapeutic substance selectively into such wounds.