Requirements definitions for complex enterprise level systems are a challenge to begin with, coupled with applying requirements development to a Commercial Off-the Shelf (COTS) component to create a customized product makes for a very interesting endeavor. In order to accomplish the customization of a COTS product, the COTS vendor is often placed under a subcontract to develop the product accordingly to written set of specifications. This seems a simple endeavor at first. However, since this COTS customization is part of a larger defense contract for the US government, things rapidly get complex very quickly. The most important item is that during the proposal phase of the US defense contract, there is not sufficient time and resources to completely define the COTS product requirements. Therefore, how do both the seller and buyer of the COTS product perform adequate requirements definition substantially prior to contract award? This presentation will articulate techniques, activities and the overall approach of a methodology to custom COTS product development during the proposal phase and afterwards during the requirements definition phase. The goal of this approach is to obtain a more complete set of COTS product customization requirements as early as possible in both the proposal (a very good DRAFT set) and requirements definition (preliminary set) phases. The goal is to arrive with as mature a set of requirements as possible at the project’s System Requirements Review (SRR) milestone. In addition, the requirements management tools that were utilized as part of this approach will be presented.