The application of foreign law is one of the most intricate aspects of private international law. This issue entails both procedural and substantive complexities. In many cases, judges opt to apply their domestic law due to the challenges of understanding foreign legal systems. This chapter delves into procedural implications, distinguishing when foreign law is treated as law or as a fact. Moreover, it outlines the Colombian legal framework, emphasizing the “mixed system” that makes both judges and parties responsible for knowing foreign law and that must be balanced by some flexible guidelines that have been provided by the Constitutional Court.