This book addresses complex real-world problems with recent techniques. It comprises selected papers from the XX. congress APDIO held in Tomar, Portugal, in July 2019. The subject of the book is the application of operational research methodologies and techniques to address complex real problems. Readers will find interesting results and applications of operational research methods and techniques in the wide variety of the addressed problems. Of particular interest are the applications of, among others, linear, nonlinear and mixed-integer programing, multiobjective optimization, metaheuristics and hybrid heuristics, multicriteria decision analysis, data envelopment analysis, clustering techniques and decision support systems, in different areas such as supply chain management, scheduling problems, production management, logistics, energy, telecommunications and finance and health. In most chapters of the book, the described problems, methods and methodologies will be illustrated by auxiliary figures and tables, as well as algorithms. The main benefit of this book is to present the latest trends in operational research methods and methodologies by top Portuguese and international researchers. The book will present a wide scope of complex real-world applications addressed by recent theoretical techniques that aim to narrow the gap between academic researchers and practitioners.