Sinusitis is one of the most common health care complaints in the United States. Approximately one in eight people in the US will have sinusitis at one time in their lives. Until recently, sinusitis has been an undertreated disease. The drastic negative effect of sinusitis on patients' quality of life has been generally underappreciated and unrecognized. Recent studies show that patients score the effects of chronic sinus disease in areas such as bodily pain and social functioning as more debilitating than diseases such as angina, congestive heart failure, emphysema, chronic bronchitis, and lower back pain, to name a few. In the past, many patients were told they would just have to "live with" their sinus problem. However, there have been enormous advances in the past 15 years in the ability to diagnose and treat these problems. This is largely because of technological advances in nasal endoscopy and X-ray imaging. Also, the development of newer, more powerful medications and significant advances in surgical treatment have played a major role in improved patient care. In this article, we provide an overview of sinusitis. We describe the modern definition of sinusitis and the normal physiology of the sinuses. We discuss the causes of sinusitis with attention to the role of allergies, acid reflux, and other related disorders. Finally, we review the current approach to the diagnosis of sinusitis.