Product line engineering has become an important and widely used approach for efficiently developing portfolios of software products. The idea is to develop a set of products as a single, coherent development task from a core asset base, a collection of artifacts specifically designed for use across a portfolio. This approach produces order-of-magnitude economic improvements compared to one-at-a-time software system development. Because the product line approach isn't limited to specific technical properties of the planned software but rather focuses on economic characteristics, high return on investment has become the anthem of the approach's protagonists. Despite this focus on ROl, most of the economic arguments for the product line community are based on singular data points derived from case studies or on reasonableness and simplistic cost curves. A manager must describe the situations in which the model can be applied. Almost all product line adoption scenarios involve fielding a set of existing or planned products as one or more product lines.