This text, now in its fourth edition, provides an understanding of the diversity and use of models used to portray the instructional development (ID) process, while reflecting the many changes in the field that have occurred since publication of the last edition in 1997. Beginning with the belief that an ID model should be selected based on the specific context of the project, a classification schema for ID models is presented that indicates whether a given model is best applied to: (1) developing individual classroom instruction; (2) products for implementation by users other than the developers; or (3) large and complex instructional systems directed at an organization's problems or goals. A schema that will help guide the way in which instructional development models are adopted or adapted is offered. The authors also present a brief history of ID models, explore numerous examples from each of the three categories in the schema, and discuss the latest trends in instructional development affecting the use of ID models. (Contains 55 references.) (MES)