Observational aspects of chromospheres and coronae in hot stars

With the advent of high resolution space observations with high sensitivity, stellar atmospheric research has entered a new phase of rapid development. All stars, and especially hot stars, are now recognized to have atmospheric characteristics that were not suspected before. All hot stars that we can observe with sufficient accuracy show chromospheres and coronae indicative of non-radiative energy fluxes as well as mass loss; these phenomena exhibit a very great range in magnitude among different stars and, in several cases, they are variable in time. These discoveries have pointed out the need for determining the atmospheric structures of hot stars and, ultimately, of determining the mechanisms responsible for the likely common origin of chromospheres-coronae and mass fluxes. This paper will focus on these observational aspects of hot stars -mainly Be stars and OB-normal stars will be treated here- and on the constraints that the observations impose upon models for these stellar atmospheres.