Since the first biologic agent was tested in the treatment of ankylosing spondylitis (AS), the ability of these therapies to dramatically improve the clinical symptoms and signs of the disease was very evident. Over the past decade, 4 tumor necrosis factor-alpha inhibitors have been approved by the Food and Drug Administration for the treatment of AS. Published data include randomized controlled trials, registries and observational studies. Guidelines have also been developed for the use of biologics in AS. Although a lot is known about the use of biologics in the AS, several "unknowns" remain. Whether these agents can alter the natural history of AS if started very early in the course or whether they can prevent extra-articular manifestations are among the important unanswered questions. Most of the data summarized in this review relate to tumor necrosis factor-alpha inhibitors, and other biologic agents that have been studied are included, as well. This review also summarizes what questions remain about the use of biologics in AS and what type of studies will be required to answer them.