As a Nature-Based Solution, urban forests deliver a number of environmental ecosystem services (EESs). To quantify these EESs, well-defined, reliable, quantifiable and stable indicators are needed. With literature analysis and expert knowledge gathered within COST Action FP1204 GreenInUrbs, we proposed a classification of urban forest EESs into three categories: (A) regulation of air, water, soil and climate; (B) provisioning of habitat quality; and (C) provisioning of other goods and services. Each category is divided into EES types: (a) amelioration of air quality; restoration of soil and water; amelioration of the microclimate; removal of CO₂ from the air; (b) provision of habitat for biodiversity; support for resilient urban ecosystems; provision of genetic diversity; and (c) provision of energy and nutrients; provision of grey infrastructure resilience. Each EES type provides one or more benefits. For each of these 12 benefits, we propose a set of indicators to be used when analyzing the impacts on the identified EESs. Around half of the 36 indicators are relevant to more than one single benefit, which highlights complex interrelationships. The indicators of wider applicability are tree and stand characteristics, followed by leaf physical traits and tree species composition. This knowledge is needed for the optimization of the EESs delivered by urban forests, now and in the future.