Invasive monitoring is a cornerstone for intensive care. Understanding iatrogenic complications arising from this invasive monitoring is obligatory for those involved in intensive care. Three basic categories of the complications of monitoring are discussed. Arterial monitoring can be complicated by thrombosis, embolism, and infection. Central venous monitoring can cause pneumothorax, arterial injury, infection, thrombosis, and air embolism. Pulmonary artery catheters can cause cardiac arrhythmias, embolism, pulmonary infarction, pulmonary hemorrhage, and cardiac injury. Knowledge of these complications is the first step toward prevention.