 This research arose the need for an even more abstract way of developing the theory of consequence relations. It has lead to a reformulation (in a category-theoretic setting) of the theory of logic systems as explained in this entry. The work has been done mainly by G. Voutsadakis in a series of papers, e.g., Voutsadakis 2002. Voutsadakis’s approach uses the notion of a pi-institution, introduced by Fiadeiro and Sernadas, as the analog of the logic systems in his category-theoretic setting. Some work in this direction is also found in Gil-Férez 2006. A different approach to a generalization of the studies encompassing the work done for logic systems and for sequent calculi is found in Galatos &amp; Tsinakis 2009; Gil-Férez 2011 is also in this line. The work presented in these two papers originates in Blok &amp; Jónsson 2006. The Galatos-Tsinakis approach has been recently extended in a way that also encompasses the setting of Voutsadakis in Galatos &amp; Gil-Férez 2017.