摘要： The loss of critical human skills that are either non-replenishable or take very long periods of time to reconstitute impacts the support of legacy systems ranging from infrastructure, military, and aerospace to information technology. Many legacy systems must be supported for long periods of time because they are prohibitively expensive to replace. Loss of critical human skills is a problem for legacy system support organizations as they try to understand and mitigate the effects of an aging workforce with highly specialized low-demand skill sets. Existing literature focuses on workers that have skills that are obsolete and therefore need to be retrained to remain employable; alternatively, this paper addresses the system support impacts due to the lack of workers with the required skill set. This paper develops a model for forecasting the loss of critical human skills and the impact of that loss on the future cost of system support. The model can be used to support business cases for system replacement. A detailed case study of a legacy control system from the chemical manufacturing industry is provided and managerial insights associated with the support of the system drawn.