Nigeria is involved in a non-international armed conflict (NIAC) against Boko Haram, a non-state armed group (NSAG) that has been causing havoc in northern Nigeria since at least 2009. In the past years, Boko Haram has developed strategically, militarily, and ideologically under the presumed leadership of Abu Shekau. Between 2013 and 2014, the group also expanded territorially - controlling large swathes of territory in Borno, Yobe and Adamawa states, - northeast Nigeria. International Humanitarian Law (IHL) is one of the oldest branches of international law, a complex and diverse area of law, covering matters such as the treatment of civilians in times of hostilities, permissible means and methods of conducting such hostilities, as well as rules vis-à-vis implementation, enforcement and accountability. Sadly, when these IHL rules are violated, or ignored, they often result in brutal and inhumane outcomes, as we have seen in the Boko Haram armed conflict. This study has examined this complex and dynamic part of international law in the context of the Boko Haram armed conflict. This study critically analysed the challenges facing the effective enforcement of international humanitarian law in the fight against Boko Haram in Nigeria. From the empirical data obtained, the study identified three fundamental issues, which largely inhibit the effectiveness of enforcing international humanitarian law in the conflict, including lack of political will on the side of the government; challenges of disciplined and trained security forces; and lastly, the issue of weak (ineffective) institutions i.e. the judiciary. Working from the tripartite challenges, the study argues that the unwillingness of the Nigerian authority in enforcing international humanitarian law (IHL) has led to several breaches of the rules of IHL during the conflict, including targeting of civilians and civilian objects, and acts of torture, rape and other forms of sexual violence. Among other issues, the thesis offers a comprehensive analysis of the causes and responses to sexual violence in the conflict. It explores the conditions that make women and girls most vulnerable to these acts by both parties to the Boko Haram conflict. Difficult questions of effective enforcement and accountability are also tackled; in particular, the political will of the Nigerian government’s obligation to investigate and prosecute alleged perpetrators of crimes in the conflict, as well as the core purpose of Common Article 1 to the 1949 Geneva Conventions, and related obligations under the laws of war so that States, concerned about the atrocities that civilians, especially women and girls in the Boko Haram armed conflict are made to endure, may find a common cause in challenging the inhumanity that is contemptuous of human beings, and the untold sorrows it imposes on millions. However, part of the process of ensuring that violations of IHL are addressed, and that people are held accountable for their actions or inactions, is to educate the society about the laws of armed conflicts, and that when violations of the rules are brought to light, some public accounting for such acts takes place. This study is part of that process, and contributes to the process of disseminating the law of armed conflict, and bringing knowledge about its content to society at large.