Book excerpt: This volume presents lecture notes of the 12th International School of Theoretical Physics held in 2016 in Rzeszów, Poland. The lectures serve as an introduction for young physicists starting their career in condensed matter theoretical physics. The book provides a comprehensive overview of modern ideas and advances in theories and experiments of new materials, quantum nanostructures as well as new mathematical methods.This lecture note is an essential source of reference for physicists and materials scientists. It is also a suitable reading for graduate students.