The notion of positive heuristic is crucial to Lakatos’s account of the growth of scientific knowledge, but his presentation of the idea is limited to a few suggestive remarks. I try to develop a more detailed account of positive heuristic, what I call model making and improving, through the work of Mark Wilson. I argue this more sophisticated account of heuristic matches well with the few things Lakatos does say on the subject and can help us see important new connections between Lakatos’s work in the philosophy of science and the philosophy of mathematics, and Wilson’s work in the philosophy of language.