Leprosy is a chronic infectious disease caused by Mycobacterium leprae. The disease presents a wide spectrum of clinical manifestations, ranging from lepromatous to tuberculoid leprosy; each form may be punctuated by episodes of acute exacerbation, called reactional states. These polar forms and reactional states appear to be determined by immunologic interactions between the host and the microorganism. This review describes the different measurable parameters that permit the classification of a particular form according to clinical, bacteriologic, histologic, and immunologic spectra. Secondly, the current state of knowledge on essential immunologic features of leprosy is presented, with a description of the various alterations of cellular and humoral immune responses that can be tested by specific and nonspecific methods. The last part of the review is devoted to an analysis of the leprosy spectrum and to speculations about a number of possible factors that may influence the immune response of the host in a manner analogous to that observed in experimental models.