Modern string theory is a rich framework which goes beyond standard quantum field theory. There is a belief that at low energies string theory reduces to the Standard Model and Einstein's gravity. In the last years we have learned that there exist different descriptions of the same physics, containing both strings and other extended objects. As a new framework, string theory inspires new approaches to old problems.

This thesis investigates a set of diverse problems: Hamiltonian analysis of extended objects; field theories with boundaries and some aspects of D-branes. Some of the topics do not have direct relevance to string theory, however all of them were inspired by string theory.

In the first part of the thesis a brief introduction to the theories of extended objects is given. In the second part we look at some examples of two dimensional quantum field theories with boundaries. These theories play a prominent role in modern string theory. In the last part of the thesis, the motivation from string theory is explained and a brief introduction to D-branes is presented.