This thesis addresses the issue of terrorism from a philosophical point of view. I focus on two questions: How should terrorism be defined? - and can it ever be morally justified? Following Grotius’ methodology on war, I interpret these questions separately, meaning that I consider that the question of what terrorism is should be treated a descriptive one, while the question of whether it can be morally justified remains a normative one. Accordingly, I begin by examining what terrorism is. This I do by analyzing six philosophical attempts to define terrorism. By extracting some core definitional elements and discussing their relevance for the subject, I propose my own definition of terrorism. This definition provides a foundation for the normative discussion on whether there can be any justification for terrorism. In this section of the thesis, I analyze three kinds of justification arguments, deriving from utilitarian, rights-based and analogy-based reasoning. On this basis, I conclude on how my findings relate to the so-called ‘war on terrorism’.