Severe sepsis is a major healthcare problem and the early initiation of antimicrobials is one of the few measures associated with improved outcomes. However, antibiotic overuse is an increasing problem in critical care. Of several potential biomarkers for antibiotic stewardship, procalcitonin represents the most widely studied and validated. In this commentary we address the current literature on the use of biomarkers to guide antimicrobial therapy in the critically ill and discuss its limitations and future directions.