Since the foundation of the ecosystem services concept in the ninetieth of the last century (Costanza et al. 1998, Costanza et al. 1997, de Groot 1992), many methods to map and assess ecosystem services have been developed and applied to policy and business questions worldwide. While many flexible methods exist at different spatial scales and ecosystem types, Jax et al. (2018) express the difficulty in choosing and applying the correct method to the right topic of interest. To enable a selection of appropriate methods, Harrison et al. (2018) developed a decision tree approach. However, Dunford et al. (2018) argue that often not a single method but a combination of methods are required for appropriate decision-making in real world situations. Thus, applying the concept of ecosystem services in practice is challenging, especially at institutional level (Saarikoski et al. 2018). This hampers comparability, applicability and transferability of ecosystem services assessments and related mapping applications across scales and European regions. It also impedes a solid overview of existing methods suitable for use at different scales in different biomes and types of ecosystems. These challenges require a consistent knowledge capitalisation infrastructure, where information is synthesised in a publicly accessible portal to enable a consistent description of different ecosystem conditions and the services they provide.<br> Going beyond the previously mentioned challenges requires a flexible methodology for assessing and mapping ecosystem services. The Horizon 2020 project ESMERALDA (Enhancing ecoSysteM sERvices mApping for poLicy and Decision mAking) developed this methodology and implemented it into the 'MAES explorer'*5 and the 'MAES Methods Explorer*1 (MME)'. The MME complements previous developments from the EU projects OpenNESS*2 and OPERAs*3. In contrast to the OPPLA*4 case-study-finder with case study areas and accompanied study area booklets and descriptions, MME focuses on methods for mapping and assessing ecosystem services and links those to selected literature and case studies. Additionally, MME provides a comprehensive and publicly searchable collection of peer-reviewed journal references and grey literature about mapping and assessing ecosystem services in Europe. This compilation is cross-related with the case study booklets produced by the ESMERALDA project and particularly methods, which are specifically used to assess and map particular ecosystem services within the case study area. Thus, searching for and filtering of particular case study areas, (related) literature references and/or methods is possible. Santos-Martin et al. (2018) provide the detailed description about the scientific procedure behind the MME tool described here.
