This is the first of four lectures on sequential games. AFter reviewing some definitions we start by talking a little bit about "theory", summed up in an acronym PDIP (who are the players, what can they do, what information/ignorance do they have, what are their payoffs). Our first games will be games of complete information (every player knows everything every other player does, each knows that the others know that, each knows that the the others know that they know.....PLUS no uncertainty inherent in the game) Then we look at how to analyze a simple 2x2 game using a neat graphical concept : the game tree.