Day surgery (DS) is continuously expanding due to both economic pressure and improvement in surgery and anesthesia. In the 1970s, only healthy patients undergoing simple procedures were accepted. Subsequent studies demonstrated that mortality and major morbidity are rare. Complicated patients are now considered suitable for DS in the current clinical practice. The aim of this article is to discuss the concept of risk evaluation in DS and to examine potentially risky situations. The outcomes that should be considered are intermediate and late outcomes, such as unplanned admission or return to hospital. Risk factors are the patient's clinical status, surgery and anesthesia and the kind of facility. Little evidence exists on what conditions should be considered predictors of adverse outcomes after DS. Non-compensated, poorly-stabilized cardiac and respiratory patients, obstructive sleep apnea, age >85 years and preterm infants are at high risk of complications. Unplanned admission or return to the hospital are more frequent after ENT and urology DS. Whether or not outpatient tonsillectomy is safe is controversial. The reported death rate per 100,000 procedures is 9.2 in offices and 0.78 in DS centers. Complicated patients need careful, time-appropriate and team-based preoperative evaluation by expert anesthetists with appropriate knowledge of DS. Patient clinical status is only one of the factors that should be considered. Surgeon's and anesthetist's skill, surgery and anesthesia technique and surgical setting are equally important. Therefore, only after evaluating their own experience and results are DS centers allowed to decide whether or not to treat a challenging patient as an outpatient.