  We survey results concerning the spectral properties of limit-periodic
operators. The main focus is on discrete one-dimensional Schr\"odinger
operators, but other classes of operators, such as Jacobi and CMV matrices,
continuum Schr\"odinger operators and multi-dimensional Schr\"odinger
operators, are discussed as well.
  We explain that each basic spectral type occurs, and it does so for a dense
set of limit-periodic potentials. The spectrum has a strong tendency to be a
Cantor set, but there are also cases where the spectrum has no gaps at all. The
possible regularity properties of the integrated density of states range from
extremely irregular to extremely regular. Additionally, we present background
about periodic Schr\"odinger operators and almost-periodic sequences.
  In many cases we outline the proofs of the results we present.
