Use of antimicrobials have been higher in Iceland than in the other Nordic countries for decates. The main reason for antimicrobial prescribing among children is acute otitis media. This book describe a study in Iceland which analyse the use of antimicrobials in pre-school children and the connection between such use and the carriage of penicillin-nonsusceptible pneumococci (PNSP). Special interesting was to show a strong association between antimicrobial usage over one year for each individual child and the risk of nasopharyngeal carriage of PNSP. Pneumococcus seems to be acquired from other children, usually in day-care settings and PNSP selectively colonised when receiving antimicrobials. Spread of novel resistant clones appears though to be the main reason for marked changes in pneumococcal resistance rates in individual communities over time. Antimicrobial drug (over)use for acute otitis media my also be associated with future episodes of otitis media and tympanostomy tube placements. Parents' expectations of antimicrobial prescriptions are associated with earlier prescriptions and antimicrobial use in the community.