This book is the eighth volume of review chapters on advanced problems of phase transitions and critical phenomena in complex systems. The aim of the book is to provide reviews in those aspects of criticality and related subjects that attract currently much attention due to essential new contributions. The current volume consists of five chapters. They discuss various aspects of studies in the field of critical phenomena as well as criticality of complex systems, where the new, emergent properties appear via collective behaviour of simple elements. Since all complex systems involve cooperative behaviour between many interconnected components, the field of phase transitions and critical phenomena provides a very natural conceptual and methodological framework for their study. As with the first seven volumes, this book is based on the review lectures that were given in Lviv (Ukraine) at the "Ising lectures" — a traditional annual workshop on complex systems, phase transitions and critical phenomena which aims to bring together experts in these fields with university students and those who are interested in the subject.