Mediums are mixed with paint both to modify the way the paint handles straight from the tube (for example, make it thinner or lengthen the drying time) and to alter the character of the paint from what you get straight from a paint tube (for example, make it transparent or opaque, gloss or matt).

The most important thing to understand about painting mediums, are the characteristics, that each component of a medium possesses, so that the artist can create a desired paint film.