  The magnetic cataclysmic variables (MCVs) present a wealth of observational
diagnostics for studying accretion flows interacting with a magnetosphere.
Spin-period pulsations from the rotation of the white dwarf are seen in optical
light, in the UV and X-ray bands, and in polarimetry, and modelling these can
constrain the size and location of the accretion footprints on the white-dwarf
surface. Tracing these back along field lines can tell us about the transition
region between the stream or disk and the magnetosphere. Further, optical
emission lines give us velocity information, while analysis of eclipses gives
spatial information.
  I discuss MCVs (particularly FO Aqr, V405 Aur, XY Ari and EX Hya, but also
mentioning PQ Gem, GK Per, V2400 Oph, HT Cam, TX Col, AO Psc, AE Aqr, WZ Sge,
V1223 Sgr and DQ Her), reviewing what observations tell us about the
disk-magnetosphere boundary. The spin-period variations are caused by a mixture
of geometric effects and absorption by the accretion flow, and appear to show
that the accretion disk feeds onto field lines differently in different
systems, being sometimes along field lines ahead of the magnetic pole and
sometimes behind the pole.
  During outbursts, when the accretion flow increases by orders of magnitude,
the disk pushes the magnetosphere inwards, and appears to feed field lines over
a much greater range of magnetic azimuth. The non-equilibrium outburst
behaviour shows an even richer phenomenology than in quiescence, adding DNOs
and QPOs into the mix.
