Due to compressor fouling, gas turbine efficiency decreases over time, resulting in decreased power output of the plant. To counteract the effects of compressor fouling, compressor on-line and off-line washing procedures are used. The effectiveness of compressor off-line washing is enhanced if combined with the cleaning of the VIGVs and the first compressor blade row by hand. This paper presents a thorough analysis of the effects of compressor on-line washing on the gas turbine performance. The analysis is based on the measured data of six gas turbines operated at two different plants. Different washing schedules and washing fluids are analyzed and compared. Furthermore, the effects of compressor on-line washing on the load distribution within the compressor are analyzed. The performance benefit of daily compressor on-line washing compared with weekly compressor on-line washing is quantified. As expected, daily compressor on-line washing yields the lowest power degradation caused by compressor fouling. Also, the effect of washing additives is analyzed. It is shown with long term data that compressor on-line washing cleans up to the first 11 compressor stages, as can be detected well in the compressor. With a view to gas turbine performance optimization, the recommendation is to perform compressor off-line washing at regular intervals and to take advantage of occasions such as inspections, when the gas turbine is cooled down anyhow. Especially for gas turbines with a high fouling rate, a daily compressor on-line washing schedule should be considered to reduce the power loss. For gas turbines operating with high fogging, compressor on-line washing has no added benefit. To determine the optimal compressor washing schedule, compressor blade erosion also has to be considered. A reasonable balance between compressor on-line washing and off-line washing improves the gas turbine performance and optimizes the gas turbine availability.