The use of computers to build diagnostic inferences is presented in two contexts: (1) closed world, exemplified by the space shuttle launch monitoring system; and (2) open world, represented by computerized diagnostic testing of reading comprehension. The analysis shows that the closed world provides a substantially cleaner environment within which to perform diagnostic inference. In the case of educational diagnosis, most domains tend to be relatively open-ended, and thus no comparable clarity can be found. If the test materials for computerized administration can be designed within tightly controlled parameters, and if the diagnostic strategy can be strongly tied to theory about performance errors within the topic domain, then many of the ambiguities of diagnostic inference will be closer to resolution. The computer has proved itself valuable in managing more traditional varieties of educational test administration and scoring. Properly programmed, the computer can become an unparalleled asset in the context of diagnostic testing. (LMO)