  A review is given for cyclotron resonant interactions in space plasmas. After
giving a simple formulation for the test particle approach, illustrative
examples for resonant interactions are given. It is shown that for obliquely
propagating whistler waves, not only fundamental cyclotron resonance, but also
other resonances, such as transit-time resonance, anomalous cyclotron
resonance, higher-harmonic cyclotron resonance, and even subharmonic resonance
can come into play. A few recent topics of cyclotron resonant interactions,
such as electron injection in shocks, cyclotron resonant heating of solar wind
heavy ions, and relativistic modifications, are also reviewed.
