Since 1993 the conference Developments in Language Theory (DLT) has been held in Europe every odd year and, since 2002, outside Europe every even year. The 13th conference in this series was DLT 2009. It took place in Stuttgart fromJune30to July3.PreviousmeetingsoccurredinTurku(1993),Magdeburg (1995),Thessaloniki(1997),Aachen(1999),Vienna(2001),Kyoto(2002),Szeged (2003), Auckland (2004), Palermo (2005), Santa Barbara (2006), Turku (2007), and Kyoto (2008). The DLT conference has developed into the main forum for language theory and related topics. This has also been re?ected in the high quality of the 70 s- missions received in 2009. Most submissions were reviewed by four Programme Committeemembersandtheirsub-referees.TheProgrammeCommitteeselected the best 35 papers for presentation during the conference. These 35 papers are also published in this proceedings volume. Members of the ProgrammeComm- tee were not allowed to submit papers. The work of the Programme Committee wasorganizedusingtheEasyChairconferencesystem,thankstoAndreiVoronkov. The conference programme included ?ve invited lectures. They were given by Mikola j Bojanczyk ' (Warsaw), Paul Gastin (Cachan), Tero Harju (Turku), ChristosKapoutsis(Nicosia),andBenjaminSteinberg(Ottawa).Wearegrateful to the invited speakers for accepting the invitation and presenting their lectures and for their contributions to the proceedings. The Informatik Forum Stuttgart provided a best paper award, which was selected by the Programme Committee. The recipient was: "Magic Numbers and Ternary Alphabet" by Galina Jiraskova.