MODELS ARE REQUIRED WHEN THE OBJECT OF STUDY IS TOO COMPLEX, MASSIVE, UNCERTAIN OR INACCESSIBLE TO BE STUDIED DIRECTLY OR IN ITS ENTIRETY. UNDER THESE CONDITIONS, A MODEL IS SOUGHT WHICH WILL EXTRACT THE ESSENCE IN A FORM AMENABLE TO ANALYSIS. THE THEORY GOES BACK TO ANCIENT ASTRONOMERS WHO DEVELOPED MODELS OF THE SOLAR SYSTEM SUFFICIENTLY RELIABLE TO DESCRIBE AND PREDICT THE PHASES OF THE MOON, TIDES, AND OTHER CELESTIAL EVENTS. AT PRESENT, OPERATIONS RESEARCH, AN APPROACH FOR DEVELOPING AND SOLVING A MODEL REPRESENTING A COMPLEX INTERACTION OF MEN AND METHODS, IS MOST COMMONLY USED. WITH OPERATIONS RESEARCH, THE SELECTION OF THE VARIABLES TO BE STUDIED AND THE INTERACTION OF THESE VARIABLES ARE CRUCIAL. TRADITIONAL METHODS OF CLASSIFYING MODELS ARE MAINLY ILLUSIONARY; QUALITATIVE VERSUS QUANTITATIVE INPUT SERVES ONLY THE MODEL BUILDER, NOT THE USER. THE MOST COMMON MODELS ARE: FORMULA MODELS (SUCH AS THE FORMULA USED BY PATHOLOGISTS TO DETERMINE TIME OF DEATH); BLACK BOX MODELS, IN WHICH A SPECIFIC ACTIVITY IS PRESUMED TO TAKE PLACE IN AN IMPENETRABLE BLACK BOX IN ORDER TO ISOLATE ACTIVITIES UNABLE TO BE STUDIED IN ANY OTHER MANNER; VENN DIAGRAM MODELS, WHICH REPRESENT INTERSECTING SUBSETS OF A DELINEATED SYSTEM; FLOW CHART MODELS, SUCH AS DESCRIPTIONS OF THE CRIMINAL JUSTICE SYSTEM; DATA MODELS, THE MOST FAMILIAR OF WHICH IS THE FBI UNIFORM CRIME REPORTS WHICH SELECTS SEVEN CRIME CLASSIFICATIONS AS AN INDICATION OF TRENDS; GRAPHS; AND OPERATIONS RESEARCH MODELS. EXAMPLES OF EACH ARE GIVEN. REFERENCES ARE PROVIDED.