In this chapter, students will learn about the concept of war and how it continues to evolve in line with the human societies that wage it. The chapter starts by examining different approaches to defining war and distinguishing it from several related activities before summarizing how most human societies developed the habit of warfare. The third section discusses how the character of contemporary warfare has changed with reference to debates about Revolutions in Military Affairs and so-called new and old wars. Finally, the chapter discusses how urban spaces, cyberspace, and outer space are becoming increasingly important domains for waging war.