That the security situation in Nigeria has become enormous and complex is no longer in doubt. To tackle insecurity, it is important to identity and investigate its sources/causes. This paper isolated and clarified the different causes/sources of insecurity in Nigeria. These sources have been attributed to quite a number of factors. The factors have been classified into external and internal. Beyond the external – internal dichotomy, sources of insecurity have also been grouped into remote and immediate. The paper identified insecurity as a major obstacle to sustainable national and educational development in Nigeria. In order to check insecurity in Nigeria, the paper recommended the following: issues of social inequity, deprivation and unemployment should be addressed, right to self-determination should be recognized, security methodology should be made proactive, there should be genuine improvement in governance and rule of law.