The Quark Matter series of conferences has the objective of reuniting world specialists of experimental and theoretical physics in the discipline of heavy ion physics. This fundamental research consists of studying excited matter at the subatomic level to understand how the constituents dynamically arrange themselves to form ordinary matter, and to understand how this organization emerged from the primordial matter created by the Big Bang at the beginning of the universe. Also to be discussed are the different leading technologies necessary for the development of more powerful experimental tools as well as material and software tools to treat the never before seen quantity of data. The Quark Matter 2015 conference is the twenty-fifth edition of this prestigious series of international conferences organized approximately every 1.5-2 years since 1980. Previous editions have taken place in Shanghai (China) 2006, Jaipur (India) 2008, Knoxville (USA) 2009, Annecy (France) 2011, Washington DC (USA) 2012, Darmstadt (Germany) planned in 2014.