The purpose of this study was to define the anatomy and dimensions of the rotator interval (RI). Thirty-two fresh-frozen cadaveric shoulders were thawed, all external musculature was removed, and coracoid osteotomy was performed to visualize the rotator cuff and RI. The 3 sides of the RI (triangle) were measured in dynamic glenohumeral positions (neutral and 45 degrees external and internal rotation). An arthroscopic posterior portal, without fluid distention, was used to visualize the RI, defects, and glenohumeral pathology. The RI was sectioned, and histologic analysis and thickness measurements were performed. The mean area of the RI was 414.46 mm(2) (95% confidence interval, 362.66-466.22 mm(2)), measured in neutral position. No statistically significant differences were measured between paired right and left shoulders; however, male specimens had a significantly larger base and area than the female specimens. The dimensions of the RI changed with glenohumeral motion. The RI is definable and dynamic with glenohumeral motion. The RI closes down with internal rotation and is opened up with external rotation. This change in the interval supports the idea that, if the arm is held in internal rotation during imbrication of the capsule, overtightening and a resulting loss of external rotation can occur.