The appearance of three comets in the autumn of 1618 touched off a controversy of such proportions that its effects are still inextricably associated with some of the most dramatic events marking the dawn of our modern era. This volume contains the principal works, in English translation, that were published during the extended controversy between Galileo and the Jesuits over the nature of comets, concluding with a commentary by Johann Kepler. The controversy of of both scientific and philosophical significance because it was in this connection that Galileo disclosed his conception of scientific method, which has been vastly influential on the course of modern thought. The principal work, Il Saggiatore (The Assayer), is also of extraordinary literary merit; it is considered the greatest polemic ever written in the domain of physical science.