  We review the physics case for very weakly coupled ultralight particles
beyond the Standard Model, in particular for axions and axion-like particles
(ALPs): (i) the axionic solution of the strong CP problem and its embedding in
well motivated extensions of the Standard Model; (ii) the possibility that the
cold dark matter in the Universe is comprised of axions and ALPs; (iii) the ALP
explanation of the anomalous transparency of the Universe for TeV photons; and
(iv) the axion or ALP explanation of the anomalous energy loss of white dwarfs.
Moreover, we present an overview of ongoing and near-future laboratory
experiments searching for axions and ALPs: haloscopes, helioscopes, and
light-shining-through-a-wall experiments.
