Besides becoming more complex, destructive, and coercive, military cyber threats are now ubiquitous, and it is difficult to imagine a future conflict that would not have a cyber dimension. This book presents the proceedings of CYDEF2018, a collaborative workshop between NATO and Japan, held in Tokyo, Japan, from 3 – 6 April 2018 under the umbrella of the NATO Science for Peace and Security Programme. It is divided into 3 sections: policy and diplomacy; operations and technology; and training and education, and covers subjects ranging from dealing with an evolving cyber threat picture to maintaining a skilled cyber workforce. The book serves as a unique reference for some of the most pressing challenges related to the implementation of effective cyber defense policy at a technical and operational level, and will be of interest to all those working in the field of cybersecurity.