Volume 1 is a collection of reprinted works of Alexei Zamolodchikov who was a prominent theoretical physicist of his time. It contains his works on conformal field theories, 2D quantum gravity, and Liouville theory.These original contributions of Alexei Zamolodchikov have a profound effect on shaping the fast developing areas of theoretical physics. His ideas are expressed lucidly, such as the recursive relation for conformal blocks and the structure of conformal bootstrap in Liouville theory, including the boundary Liouville theory. These ideas are at the foundation of the subject and they are of great interest to a wide community of physicists and mathematicians working in diverse areas.Volume 2 includes Alexei Zamolodchikov's works on non-perturbative methods in 2D quantum field theory, and on integrable models.He was the originator of a powerful technique known as the Truncated Conformal Space Approach. Today, the application of this technique is a standard way of analyzing 2D quantum field theories beyond perturbation theory. Equally profound was his analysis on the structure of operator product expansions. His idea of isolating non-perturbative contributions in terms of one-point expectation values is of fundamental importance to understanding correlation functions. Pioneering works of Alexei Zamolodchikov in integrable quantum field theories are also included in this volume. His works on factorizable scattering matrices and applications of the Thermodynamic Bethe Ansatz in integrable quantum field theories are classics. Overall, his profound ideas have continuously shaped, to a large extent, our contemporary understanding of 2D field theories, and quantum field theory in general. They will be of great interest to a wide community of physicists and mathematicians working in diverse areas.This 2-volume collection of remarkable research papers can be used as an advanced textbook by graduate students specializing in string theory, conformal field theory and integrable models of QFT. It is also highly relevant to experts in these fields.