This article addresses the relationships between the principles of universal jurisdiction and complementarity and their difficulties of implementation. Even if the two principles are well known, there are still a number of obstacles - legal and non-legal - to proper and better implementation. Moreover, universality and complementarity are quite often to be applied in a difficult political environment, keeping in mind that these principles have to deal with international and national constraints. The number of obstacles is such that the two principles face many challenges. This article advocates that the principle of complementarity represents one aspect of the principle of universality and should rely on its general acceptance to further its efficiency and implementation. In conclusion, the article explores some possible ideas to be developed to reach this goal.