The purpose of this thesis is to explore the nature of military behaviour during the High Middle Ages, in what is normally called the Age of Chivalry. I am not entirely comfortable with this appellation, which is why I have chosen to discuss the ethos and practice of warfare. My focus is essentially on the societies of north-western Europe which displayed certain characteristics in warfare, which they exported into the Mediterranean region and further east. It is somewhat of a simplification to describe this military culture as that of 'knight and castle'; but it is a convenient starting point. In what follows I have drawn together fourteen of my published articles over the period 1982-2005, in order to present my interpretation of the main strands that can be identified in warfare between 1050 and 1250. Although I continue to be research active and have published, or I am still in the process of publishing, half-a-dozen articles over the last five years, I felt that I could best present a coherent thesis from the pieces which I have selected.