Operational managers of wind turbines usually monitor a big fleet of turbines and thus need highly condensed information to identify underperforming turbines and to prioritize their work. Key performance indicators (KPIs) are a solid and frequently used tool for this purpose. However, the KPIs used in the wind industry are not unified to date, which makes comparison in the industry difficult. Further, comprehensive standards on a set of KPIs for the wind industry are missing. This article identifies and recommends KPIs and provides detailed definitions to make KPIs comparable and to enable benchmarking. The starting point of this work is an industry survey with 28 participants intended to identify commonly used KPIs, collect various possible definitions, and prioritize them. Out of a total of 50 KPIs, we discuss in a next step 33 selected KPIs on performance, maintenance, and reliability in detail and recommend definitions, most of which are based on international standards. As a result, operators can easily use these recommendations to base their system of KPIs. By using this unified set of KPIs, operators can be well-prepared to conduct industrywide comparisons and benchmarks. The survey and this article will also serve as a basis for committee work of the FGW e.V. to develop a corresponding technical guideline.