  ER UMa stars are a recently recognized small subgroup of SU UMa-type dwarf
novae, which are characterized by the extremely high outburst frequency and
short (19--48 d) supercycles. From the current thermal-tidal disk instability
scheme, they are considered to be high mass-transfer SU UMa-type dwarf novae,
and comprise a link to "permanent superhumpers" below the period gap. They do
not only provide an opportunity to test the applicability of thermal-tidal
instability model but also pose problems on the origin of high mass-transfer in
short orbital-period cataclysmic variables. A historical review of this
subgroup and recent topics of ER UMa stars, the unique pattern of superhump
evolution and the "helium ER UMa analog" (CR Boo), are also discussed.
