Since the last update of the European strategy on particle physics (ESPP) the
interest in hypothetical very weakly interacting slim particles, dubbed WISPs,
has gained significant momentum. Searches for WISPs with masses below
about 1 eV require new approaches beyond accelerator experiments. This document
summarizes the physics case, the experimental status and its prospects
for the coming 10-20 years. Its focus is on larger scale experiments with European
leadership requiring a more strategic approach for their potential realization.
This document will be submitted in December 2018 as an input to the
update process of the European Strategy on Particle Physics (ESPP).