Pain is one of the most common complaints expressed by hospital patients and is the main reason they seek medical help. Pain is always subjective, so its severity should be assessed individually for each patient. The main issue with pain management in children is the difficulty involved in evaluating it. Numerous studies have developed tools that would allow for an accurate assessment of the intensity of pain in children in the postoperative period. Adequate postoperative pain assessment in pediatric patients may significantly improve their comfort and quality of life. Postoperative pain prolongs recovery and hospitalization; therefore, the severity of the pain should be part of a routine assessment. Whichever tool is applied to measure pain, it should take into account the child’s age, language, ethnicity, and cognitive ability. There is no one universal method for pain assessment which is appropriate for every pediatric patient. This article provides a review of the available subjective methods of postoperative pain assessment, including new objective diagnostic methods and the latest guidelines for postoperative pain therapy in a group of pediatric patients.