In 1921, Ludwig Wittgenstein (1889-1951) published a landmark book in philosophy: the Tractatus Logico-Philosophicus, usually called just the Tractatus. It puts forth a profound theory-now called the picture theory-about the meaning of language. Simply put, the Tractatus aims to answer the question "What can be expressed by language?" It is an understatement to say that the Tractatus is difficult to read and understand. Accordingly, numerous introductory books and commentaries have been written. These texts are important contributions in philosophy but also are quite complex, even when described as being introductory. In contrast, this book brings out the fundamental ideas of the Tractatus in simple, everyday language, with all concepts and conclusions illustrated by numerous examples taken from the natural sciences. This book is also available in German: Hülster Einführung in Wittgensteins Tractatus Logico-Philosophicus.