In the first part of this book we have discussed (super-)string theory in flat 26(10)- dimensional backgrounds. It is clear that such string theories are phenomenologically not relevant. Our main task in the second part is to understand some of the technology that is used to build string theories that are relevant, in particular for 4-dimensional physics. The basic idea is to study the motion of superstrings on backgrounds in which some of the dimensions are curled up. This process is known as compactification. As we shall discuss, describing strings in curved backgrounds is a highly non-trivial task. We shall sketch a few of the basic technologies in the next three chapters. This will be followed by two more blocks of chapters, one on Calabi-Yau compactifications, the other on string dualities, with special emphasis on the AdS/CFT correspondence and its application to gauge theory.