Many surgical patients spend time in an intensive care unit and on the wards. Medication errors are one of the most frequent causes of preventable harm in ICU and wards, and occur predominately in the dispensing and administration phases. Despite a plethora of publications and studies of medication errors in these locations, significant differences in definitions, study methods, and local workflows lead to remarkable differences in the rates of error reported. Increasing number of medications per patient, nurse to patient ratios, and patient age contribute to medication errors in the ICU. Pediatric ICUs tend to have higher rates of error, and this often is due to mis-calculation of a dilution or a weight based dose. On the wards, administration errors are common, especially when exact timing is expected for a given administration. Prescribing errors are often the result of lack of knowledge, whether related to diagnosis or medication; administration errors are more frequently skill based errors such as slips or lapses.