We discuss how soft X-ray transients exhibit light curves associated with two accretion-disc instabilities that are well known to operate in cataclysmic variables. A thermal instability that is due to hydrogen ionization leads to the transient nature but is modified by X-ray irradiation of the disc. Growth of the disc between the ensuing outbursts leads to a 3:1 commensurability instability at the outer edge of the disc that drives superoutbursts and produces superhumps in the lightcurve if the mass ratio is sufficiently small. Superhumps are observed only in the short period SU UMa CVs but black hole primaries allow SXTs to exhibit superoutbursts and superhumps at much larger periods. We point out similarities between SXT superhump shapes and those of the small class of ER UMa CVs, The similarity between these systems stems from the fact that the mass-transfer rates are almost large enough to stabilize the disc against the thermal instability in both cases.