Abstract : Diesel engines are well known for their operational robustness and efficient performance. These attributes make them a leading choice for prime movers in critical DoD, industrial, and mobility applications. Despite the diesel engine's known reliability, there are some operational issues that justify monitoring critical engine components and subsystems in order to increase the overall availability and readiness of diesel-powered systems. Moreover, engines typically constitute a significant fraction (1/10-1/5) of the acquisition cost and a comparable fraction of the life cycle cost for mobility applications (trucks, armored vehicles), thereby providing the motivation for engine condition monitoring on the basis of reducing life cycle costs. Review of the available literature indicates that the fuel injection and cooling subsystems are among the most problematic on diesel engines contributing to reduced readiness and increased maintenance costs. These faults can be addressed and studied using scaled testing to build the necessary knowledge base to quickly transition the methods to full-scale, more costly diesel engines. Towards this goal, a Diesel Enhanced Mechanical Diagnostics Test Bed (DEMDTB) has been developed that uses an array of sensors to measure pressure, temperature, vibration, and displacement. The test bed is used for experimental collection of healthy, seeded fault, and transitional fault test data from the diesel engine and driveline components. The data is analyzed with time and frequency based analysis methods to characterize 'healthy' and 'faulty' operation. The purpose of this paper is to present an overview of previous research conducted for diesel engine diagnostics, discuss recent diesel engine diagnostics developments, and to lay the basis for straightforward concept designs for practical diesel engine monitoring/diagnostics systems that will enable system prognostics.