 By integrating the complementary perspectives of qualitative logic and numerical probability theory, probability logics are able to offer highly expressive accounts of inference. It should therefore come as no surprise that they have been applied in all fields that study reasoning mechanisms, such as philosophy, artificial intelligence, cognitive science and mathematics. The downside to this cross-disciplinary popularity is that terms such as ‘probability logic’ are used by different researchers in different, non-equivalent ways. Therefore, before moving on to the actual discussion of the various approaches, we will first delineate the subject matter of this entry.