The diagnosis of allergy to penicillins is based on a positive history and cutaneous tests. The sensitivity of these tests is around 70%, therefore it is sometimes necessary to supplement them with in vitro tests, more rarely with drug provocation challenges. Among patients allergic to penicillin, 1 to 2% will develop an allergy to cephalosporins in the event of exposure to this antibiotic class. In this paper, we describe options for investigating and treating allergy to penicillins and cephalosporins.