Bottom-up emulations of real sustainment systems that explicitly model spares, personnel, operations, and maintenance are a powerful way to tie funding decisions to their impact on readiness, but they are not widely used. The simulations require extensive data to properly model the complex and variable processes involved in a sustainment system, and the raw data used to populate the simulation are often scattered across multiple organizations. IDA has worked with military and civilian sponsors to construct such end-to-end simulation models for maritime and aviation weapon systems and to investigate the strategic levers that drive readiness. In this conference presentation, we describe the complexities of properly emulating sustainment concepts and argue for the urgency of increased end-to-end modeling efforts in improving readiness across weapon systems.