Infection developing in the intensive care unit is a common complication of critical illness, but notoriously difficult to diagnose. A definite diagnosis based on the most reliable tests usually is not possible for at least two days. It is unclear what the optimal management approach should be while awaiting the results of diagnostic tests. In some circumstances, broad spectrum antibiotics are started with a plan to adjust them once the results of cultures are available. Observational studies show that this results in greater antibiotic use, and the risk of superinfection and resistance. In other circumstances, antibiotics may be withheld pending the results of cultures, a strategy that leads to a delay in therapy when cultures are positive, and that may be associated with a worse clinical outcome.