The aim of this lecture is to make an introduction to phase transitions. Afetr some examples of the use of, or prevention from, phase transitions in technological applications (ch.1), a survey is made of microscopic interactions that can lead to phase transitions in a range of temperature usefull for technological purpose (ch2).  An insight of the cooperative phenomena is presented (ch.3) as well as some universal features of phase transitions (ch.4). Finally, the Landau phenomenological appraoch is presented for 2nd and 1st order phase transitions (ch.5). Determination of the correlation length from diffuse scattering experiment is also given as appendix.