Hot subdwarfs are helium burning stars, located at the blue end of the horizontal branch. Although most of these stars must have lost their hydrogen envelope by stellar winds or through one or two common-envelope phases, some of them are still likely to have a very thin hydrogen envelope. Envelope ejection is the main reason for these stars being hydrogen poor. Generally their mass is between 0.4M  and 0.8M  and radius is around 0.1R. Approximately 50% of hot subdwarf stars are in binary systems with periods shorter than 30 days. Their companions are generally white-dwarfs (WD) or M dwarfs but there are also some systems with unseen components. Binary systems contain many different types of stars and their evolution may follow different paths ending up with different results and producing many types of exotic stars. In this talk we will go through the extra-ordinary properties of hot subdwarfs and their companions in binary systems.