 Although the regress problem is not a central contemporary issue, it is helpful to explain coherence theories as responses to the problem. This will also serve to illustrate some challenges that a coherence theory faces. We will then turn to the concept of coherence itself as that concept is traditionally conceived. Unfortunately, not all prominent authors associated with the coherence theory use the term coherence in this traditional sense, and the section that follows is devoted to such non-standard coherence theories. The arguably most systematic and prolific discussion of the coherence theory of justification has focused on the relationship between coherence and probability. The rest of this entry will be devoted to this development, which took off in the mid-1990s inspired by seminal work by C. I. Lewis (1946). The development has given us precise and sophisticated definitions of coherence as well as detailed studies of the relationship between coherence and truth (probability), culminating in some potentially disturbing impossibility results that shed doubt on the possibility of defining coherence in a way that makes it indicative of truth. More precise descriptions of key entailments of these results, and ways to address the worries that they raise, will be discussed in later sections of this entry.