Introduction to string field theory

This article provides an introduction to string field theory. It is focused on a gauge invariant, bosonic open string field theory. This has the advantage of being general enough to illustrate many of the new features of string field theory, while being specific enough to keep the analysis simple. It is also one of the most developed theories at the moment. Even with this restriction, there are several actions which have been proposed. Much of the general discussion will apply equally well to all candidate theories, however when it is necessary to become more specific, it focuses on an approach due to Witten and the related purely cubic action