We are often presented with arguments designed to convince us to believe certain things, or to act in certain ways. Most of the time we do pretty well at sorting out the bad arguments from the good ones, but what exactly are the grounds on which we do this? One criterion is surely that an argument should be valid, that is, that its conclusion should follow from its premises. But just what is it for an assertion to follow from others? In this course we will develop a formal framework within which validity, along with other central concepts of deductive logic, can be rigorously defined and studied.