This is a short introductory note to the texts of lectures presented at a Royal Society Discussion meeting held on 14-15 Febrary 2005 and now published in this issue of Philosophical Transactions A.

where N is the number of selected events, N BG is the number of background events, f ξ < 5 × 10-6 is the fraction of events that pass the event selection but have ξ<5×10 −6, is the integrated ...