 Besides the model-theoretic side, axiomatic investigations of truth and related paradoxes have become increasingly important since the seminal papers of Friedman and Sheard 1987, Feferman 1991. Since the year 2000, this research thread has been intensively studied with various aims, from proof-theoretic analysis to philosophical discussion of minimalism (for a survey of the varieties of truth theoretic systems and appropriate references, see the entry on  axiomatic theories of truth and  the recent monographs of Halbach 2011, Horsten 2011; see also the papers Feferman 2008, Fujimoto 2010, Leigh and Rathjen 2010).