Book excerpt: This book is the seventh volume of review chapters on advanced problems of phase transitions and critical phenomena, the former six volumes appeared in 2004, 2007, 2012, 2015, 2018, and 2020. The aim of the book is to provide reviews in those aspects of criticality and related subjects that are currently attracting much attention due to essential new contributions.The book consists of five chapters. They discuss criticality of complex systems, where the new, emergent properties appear via collective behaviour of simple elements as well as historical aspects of studies in the field of critical phenomena. 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 the first six 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.