Close behind; closely following. Although these two clichés are very similar, they are not wholly interchangeable. To be at someone’s heels is to be immediately behind, with the implication of chasing or otherwise harrying the person. To be on the heels of someone (or something) means to be following in quick succession (but not necessarily catching or overtaking). Both terms conjure up the idea of a dog being at one’s heels, and both are quite old. John Gower wrote in 1390, “There bene also somme as men sale that folwen Simon ate heles.” And Shakespeare wrote, “One woe doth tread upon another’s heel, so fast they follow” (Hamlet, 4.7).