Model-based simulation systems have been created in various fields of engineering to train personnel or students in operation, maintenance and troubleshooting of complex devices and systems. A review of literature indicates a lack of good overviews of the approaches to system design of model-based training simulations in Intelligent Computer Assisted Instruction (ICAI). Though single systems have to some extent been evaluated with regard to their performance, an organised evaluation, especially a comparative evaluation of the systems that have been created within the field is lacking. To be able to successfully conduct an in-depth review under these conditions, we concentrate and centre our review around the design of the domain simulator component and its implications for the other parts of instructional system. For this purpose we have identified features that are in our opinion suitable to characterise the design approach of a model-based simulation in ICAI for training. We have organised these design features in a framework to enable an effective review. Within this framework, we investigate each feature separately and illustrate it by giving examples from existing applications where appropriate.