  Ultra-light scalar fields, with masses of between m=10^{-33} eV and
m=10^{-22} eV, can affect the growth of structure in the Universe. We identify
the different regimes in the evolution of ultra-light scalar fields, how they
affect the expansion rate of the universe and how they affect the growth rate
of cosmological perturbations. We find a number of interesting effects, discuss
how they might arise in realistic scenarios of the early universe and comment
on how they might be observed.
