There exist strong observational links between ion cyclotron emission (ICE) and fusion reactivity in tokamak plasmas. These links originally emerged from deuterium discharges in the Joint European Torus (JET) and were demonstrated most recently in the Preliminary Tritium Experiment. They include the proportionality of ICE intensity to measured fusion reactivity over six decades in signal intensity; correlations in the time evolution of the ICE signal and neutron flux during discharges; the matching of the spectral peak frequencies to successive local ion cyclotron harmonics at the outer midplane edge; and correlations between ICE and the observed impact of magnetohydrodynamic activity, such as sawteeth and edge-localized modes, on energetic ions. The observations are broadly consistent with the excitation of the fast Alfvén wave through cyclotron resonance with the local non-Maxwellian fusion product population — the so-called magnetoacoustic cyclotron instability. The theory of this instability is extended to the regime of arbitrary k||, in which it is necessary to include both wave-particle cyclotron damping and the positive-energy loading due to resonant cyclotron harmonic waves supported by the thermal ions. The consequences of arbitrary k|| for the instability thresholds are described. An outline is given of the close similarities between ICE from tokamaks and signals at multiple ion cyclotron harmonics observed in the Earth's magnetosphere, which apparently originate from regions where there is a ring-type population of energetic protons. This emission also appears to be explicable in terms of the magnetoacoustic cyclotron instability, and comparison with tokamak observations yields information on the distinction between features generic to the emission mechanism and those specific to particular magnetic geometries.