Directing his polemics against the pedantry of his time, Galileo, as his own popularizer, addressed his writings to contemporary laymen.His support of Copernican cosmology, against the Church’s strong opposition, his development of a telescope, and his unorthodox opinions as a philosopher of science were the central concerns of his career and the subjects of four of his most important writings. Stillman Drake’s introductory essay places them in their biographical and historical context.Included here are “The Starry Messenger” (1610), “Letter to the Grand Duchess Christina” (1615), and excerpts from “Letters on Sunspots (1613) and “The Assayer” (1623).