For every invention, a scientific theory is being put to practical use. Sometimes the theory is explored first, sometimes the invention gives rise to the formulation of the theory. But it is the theoretical formulation which allows for a broad practical application, limited not to one but to many inventions. In this text the theories behind the steam engine, the electric light, theories of personality, Black's chemical investigations, Sillman's work in conjunction with Goodyear and others, theories which led to medical cures, and many others are explored in terms of the men who made them and their application in the field of invention. Many specific scientific facts are revealed here in a text for young scientists which affords him an accurate perspective as well as an appreciation of what theoretical work has laid the groundwork for his studies.