Model theory investigates the relationships between mathematical structures ("models") on the one hand and formal languages (in which statements about these structures can be formulated) on the other. Examples of these structures are the natural numbers with the usual arithmetical operations; the natural numbers with the usual arithmetical operations; the structures familiar from algebra; and ordered sets. The emphasis in this book is on first-order languages, whose model theory is best known. An example of a result is Lowenheim's theory (the oldest in the field): a first-order sentence true of some uncountable structure must hold in some countable structure as well. The author deals with second-order languages and several of its fragments as well. As the title indicates, this book introduces the reader to what is basic in model theory. A special feature is its use of the Ehrenfeucht game by which the reader is familiarized with the world of models.