Book excerpt: This is a collection of lectures by 11 active researchers, renowned specialists in a number of modern, promising, dynamically-developing research directions in condensed matter/solid state theory. The lectures are concerned with phenomena, materials and ideas, discussing theoretical and experimental features, as well as with methods of calculation.Readers will find up-to-date presentations of the methods of carrying out efficient calculations for electronic systems and quantum spin systems, together with applications to describe phenomena and to design new materials. These applications include systems of quantum dots, quantum gates, semiconductor materials for spintronics, and the unusual characteristics of warm dense matter.