This is the 5th, revised and updated, edition of Seapower: A Guide for the Twenty-First Century. The sea has always provided the basis for mankind's prosperity and security. With the emergence of an increasingly globalised world trading system in the early part of the 21st century, this became ever more obvious. Contemporary conditions have therefore required navies to adapt and to expand the range of their interests, activities and responsibilities. The Houthi assault on international shipping in the Red Sea and the need to monitor sanctions and ‘dark fleet’ operations shows that these tasks are important and developing fast. Even so, and especially with the re-emergence of potentially de-stabilising great power competition, traditional hard power naval concerns still predominate. This book investigates the consequences of all this for the developing nature, composition and functions of the world's navies, and provides a guide for the changing but crucial role of seapower in the second quarter of the 21st century. This edition contains new material on the Russo-Ukraine and Gaza wars, tensions in the Indo-Pacific, the Arctic and North Atlantic, the emerging concept of ‘seabed control,’ as well as the extent that technological innovations and other recent strategic developments are challenging traditional concepts of maritime strategy and the roles of navies. This book is essential reading for students and practitioners of naval power, maritime security and naval history, and highly recommended for students of strategic studies, international security and International Relations.