Computer-Aided Trainers (CATs) for mission avionics subsystems on the E-3A "Sentry" aircraft simulate BITE outputs to support self-paced practice in flight line maintenance procedures. They represent a new breed of CATs evolved through application of the Instructional System Development (ISD) process and a unique synthesis of hardware, software and "courseware" to satisfy organizational maintenance training requirements. This paper is in two parts. Part I summarizes the application of the ISD process used to identify requirements for training and training equipment. Part II concentrates on the evolution of the maintenance trainers, emphasizing the system development using as a case study the E-3A Maintenance Procedure Simulator (MPS) for the AN/ASN-118 Navigation Computer System.