This look at Gassendi’s philosophy and science illuminates his contributions to early modern thought and to the broader history of philosophy of science. Two keys to his thought are his novel picture of acquiring and judging empirical belief, and his liberal account of criteria for counting empirical beliefs as parts of warranted physical theories. By viewing his philosophical and scientific pursuits as part of one and the same project, Gassendi’s arguments on behalf of atomism can be fruitfully explained as licensed by his empiricism.