X-ray bursts: nuclear flash model and its difficulties.

X-ray bursts are considered to be produced by the nuclear flash taking place in the accretion layer of a neutron star (type I) or by accretion instabilities which feed matter suddenly on the surface of a neutron star. A brief account of the nuclear flash model and the main consequences thereof are given. Effects of general relativity on the nuclear flash and the observation of X-ray bursts are reviewed. A number of burst properties observed mainly by Hakucho can be hardly accounted for in terms of the nuclear flash model.