String theory is the most promising candidate for the theory unifying
all interactions including gravity. Since it has an extremely difficult
dynamics, many important insights have come from the study of its
low-dimensional versions. A particularly interesting case is 2D string
theory, which has a very rich structure, but at the same time is
solvable. Its complete solution in the simplest linear dilaton
background was obtained using its representation as Matrix Quantum
Mechanics, which provides a very powerful technique and reveals the
integrability hidden in the usual CFT formulation. This book presents a
review of these results and extends them to non-trivial backgrounds of
2D string theory. It shows how MQM incorporates the perturbations
changing the background, their integrability, and various results on
correlation functions, thermodynamic behavior, structure of the target
space and non-perturbative effects in the deformed background. The book
is recommended for students and researchers working in theoretical and
mathematical physics.