Surgical intervention remains a trauma for patients even given modern techniques and the use of highly specialized instruments. Any surgical maneuver can cause injuries and complications. The severity of such injuries is determined by the disease itself, the patient's comorbidities and the surgeon's experience. In this chapter, we present the iatrogenic complications arising following major surgeries performed openly or laparoscopically. In the first section, general complications, such as hemorrhage, fever, ileus and abscesses, are presented. Next, organ-related complications occurring during laparotomy, drainage of the abdominal cavity or abdominal wall reconstruction are described in detail. We then turn to the specific intra- and postoperative complications of various organs’ surgical treatment due to erroneous surgery indications, vascular lesions and lesions of the neighboring organs. We describe complications in the surgery of the mediastinum, thoracic and abdominal cavity and describe the particularities of thyroid, lung, esophagus, stomach, small intestine, appendix, colon, pancreas, hepatobiliary tract and hernia surgery and postoperative parietal defects. Complications occurring as a result of laparoscopic intervention, erroneous indications or complications in performing pneumoperitoneum during insertion of the trocars are also presented. Finally, we consider the specific iatrogenic complications arising during minimally invasive bile, hiatal hernia, cardiac achalasia, spleen and morbid obesity (gastric sleeve and gastroplication) surgery.