The Lecture Notes collect seven mini-courses presented at the 5th Prague Summer School on Mathematical Statistical Physics that took placeduringtwoweeksofSeptember2006.Aswithprecedingschools, it was aimed at PhD students and young postdocs. The central theme of the volume is what could be called "mathematics of phase transitions" in diverse contexts. Even though all courses were meant to introduce the reader to recent progress of a particular topic of modern statis- cal physics, attention has been paid to providing a solid grounding by carefully developing various basic tools. One of the techniques that led, more than two decades ago, to a seriesofimportantoutcomesinthetheoryofphasetransitionsoflattice models was re?ection positivity. Recently it resurfaced and was used to obtain interesting new results in various settings. The lectures of Marek Biskup include a thorough introduction to re?ection positivity as well as a review of its recent applications. In addition, it contains a crash course on lattice spin models that is useful as a background for other lectures of the collection. Also the following two contributions concern equilibrium statistical physics.ThelecturesofDmitriIo?earedevotedtoastochasticgeom- ricreformulationofclassicalaswellasquantumIsingmodels.Auni?ed approachtotheFortuin-Kasteleynandrandomcurrentrepresentations in terms of path integrals is presented. Statistical mechanics of directed polymers interacting with o- dimensionalspatiale?ectsisatopicwithvariousapplicationsinphysics and biophysics. The lectures of Fabio Toninelli are devoted to a th- ough discussion of the localization/delocalization transition in these models.