Comparative literature assessing the relative safety and efficacy of different BoNT products is limited. The quantity and quality of data vary by preparation and indication. Clinicians seeking data relevant to the care of patients with specific conditions may find only reports about small numbers of patients with varying symptoms. While a literature search for "botulinum neurotoxins" will yield a large number of publications; only a fraction of these meet criteria for an academic evidence-based review. Patients may have been treated with a different BoNT formulation than that with which the physician is familiar, or there may be little or no clinical data on the use of a specific BoNT product for the proposed intervention. This paper is an introduction to a series of papers (which follow) in which an expert panel reviewed the BoNT clinical trial literature in order to provide evidence-based recommendations regarding the clinical use and efficacy of available BoNT preparations for four major therapeutic areas: movement disorders, spasticity, urology, and secretory disorders. Expert opinion is also included to address practical issues where more evidence and further research is needed.