The main reason why Blue Phases have no practical application is related to their very limited existence range. Indeed, they are always inserted between the liquid isotropic phase and the cholesteric phase in a very narrow temperature interval (fig. B.VII.5). This marginal position in the phase diagrams is also a possible explanation for the little interest they aroused for more than half a century, until the English chemist George Gray (well known for synthesizing the cyanobiphenyls) studied them again (after Reinitzer, Friedel and Lehmann) about 1956 and noticed an anomalous texture very close to the clearing point of the cholesteric phase [1a]. Later on, Gray and his coworkers, Coates and Harrison, refined these initial observations and concluded that not one but three different textures were present between the isotropic and cholesteric phases [1b-d]. These observations aroused the interest of the scientific community and prompted new research. This chapter summarizes the results of this research, mainly on Blue Phases I and II.