 In an intellectualist recidivist mode, this entry will outline his investigations in physics and astronomy and exhibit, in a new way, how these all cohered in a unified inquiry. In setting out this path, we shall show why, at the end of his life, Galileo felt compelled (in some sense of necessity) to write the Two New Sciences, which stands as a true completion of his overall project and is not just a reworking of his earlier research that he reverted to after his trial, when he was under house arrest and going blind. Particularly, we shall try to show why both of the two new sciences, especially the first, were so important—a topic not much treated except recently (Biener 2004; Raphael 2011). In passing, we shall touch on his methodology and his mathematics, and here refer you to some of the recent work by Palmieri (2001; 2003). At the end, we shall add some words about Galileo, the Catholic Church, and his trial.