The various Electron Spin Resonance methods that have been used for st
udying transient free radicals are described, and their limitations an
d advantages discussed. Continuous wave and Fourier Transform time-res
olved methods are compared and shown to be complementary in studies of
radicals produced using flash photolysis. The role of Chemically Indu
ced Dynamic Electron Polarization (CIDEP) in determining reaction path
ways and in linking the photochemistry and photophysics of molecules i
s described as is the role played by Reaction Yield Detected Magnetic
Resonance (RYDMR) and magnetic field effect (MFE or MARY) studies in u
nderstanding the reactions of radicals.