Prior to the launch of the Ariel V satellite in 1974, four transient X-ray sources had been observed. We placed the study of sources of this type amongst the major objectives of the satellite, and so ensured that the spacecraft ground centre was capable of quick reaction, and that experimenters had rapid access to their data, so that new transients could be rapidly recognised, and an observing programme for them produced and carried out. This has been very successful - in its first 18 months of operation the satellite made observations of at least 14 transients.
Of these transients, five form a rather well defined class whose properties are rapidly becoming clearer. This class I shall call the “classical” transient. The remainder form a much more miscellaneous collection which cannot be clearly distinguished from the normal X-ray sources. There is some indication that the classical transients include two species, so to start with I shall describe one of each which has been particularly well-observed by Ariel V and SAS-3.