  Experiments looking for rare events like the direct detection of dark matter
particles, neutrino interactions or the nuclear double beta decay are operated
deep underground to suppress the effect of cosmic rays. But the production of
radioactive isotopes in materials due to previous exposure to cosmic rays is an
hazard when ultra-low background conditions are required. In this context, the
generation of long-lived products by cosmic nucleons has been studied for many
detector media and for other materials commonly used. Here, the main results
obtained on the quantification of activation yields on the Earth's surface will
be summarized, considering both measurements and calculations following
different approaches. The isotope production cross sections and the cosmic ray
spectrum are the two main ingredients when calculating this cosmogenic
activation; the different alternatives for implementing them will be discussed.
Activation that can take place deep underground mainly due to cosmic muons will
be briefly commented too. Presently, the experimental results for the
cosmogenic production of radioisotopes are scarce and discrepancies between
different calculations are important in many cases, but the increasing interest
on this background source which is becoming more and more relevant can help to
change this situation.
