AUTHORS' SUMMARYTechnological knowledge, like all knowledge, can be learned or accumulated, but it can also be lost or 'un-learned'. In other words, knowledge capital depreciates. This case study considers the different sources of knowledge depreciation including reasons why knowledge is lost (e.g., due to staff turnover) as well as why knowledge becomes obsolete (e.g., due to rapid innovation). Estimates of typical knowledge depreciation rates are reviewed, and the limited examples related to energy technologies are discussed in more detail. Knowledge depreciation rates reach 100%/year in service industries, characterized by high staff turnover. In the energy technology field, knowledge depreciation rates of between 10%/year (wind turbines) and 30%/year (solar PVs) have been identified. Illustrative calculations based on public energy R&D statistics of IEA countries show the implications of knowledge depreciation for two groups of energy technology innovations: nuclear power and energy efficiency. A conclusion is that a stable, gradually rising trajectory of policy support is as important, if not more important for mitigating knowledge depreciation than the absolute level of policy support if characterized by "boom and bust" cycles. With knowledge depreciation, continuous knowledge "recharge" becomes critical, e.g., through stable R&D efforts and sustained market formation incentives.