The incidence of deep neck space infections has dramatically decreased since the advent of antibiotics, but with delayed treatment they carry the potential for significant morbidity and mortality. Odontogenic infections with involvement of the submandibular space are the most common source of deep neck space infections in adults, whereas in the pediatric population the most common cause is acute tonsillitis with involvement of the peritonsillar space. The newest group of patients at risk for deep neck space infections are intravenous drug abusers who inject the major vessels of the neck. Knowledge of neck spaces and fascial relationships is important in understanding the presentation, treatment, and complications of deep neck space infections. The spaces, which are created by various fasciae of the head and neck, are only potential spaces in that under normal conditions they cannot be examined clinically or radiographically. As the spaces are invaded by bacteria, a cellulitis or abscess occurs, and this infection may travel through paths of least resistance from one space to another.