Macroalgae (or seaweed) aquaculture can potentially provide many ecosystem services, including climate change mitigation, coastal protection, preservation of biodiversity and improvement of water quality. Nevertheless, there are still many constraints and knowledge gaps that need to be overcome, as well as potential negative impacts or scale-dependent effects that need to be considered, before macroalgae cultivation in Europe can be scaled up successfully and sustainably. To investigate these uncertainties, the Expert Working Group (EWG) on Macroalgae was established. Its role was to determine the state of knowledge regarding the potential of macroalgae culture in providing climate-related and other ecosystem services (ES) and to identify specific knowledge gaps that must be addressed before harvesting this potential. The methodological framework combined a multiple expert consultation with Delphi process and a Quick Scoping Review (QSR). To analyse the outcome of both approaches, the EWG classified the findings under the categories Political, Environmental, Social, Technical, Economic and Legal (PESTEL approach) and categorised the ES based on the CICES 5.1 classification. Although representative stakeholders from many different disciplines were contacted, the majority of responses to the Delphi process were from representatives of academia or research. While the results of each method differed in many ways, both methods identified the following top six ecosystem services provided by seaweed cultivation: i) provisioning food, ii) provisioning hydrocolloids and feed, iii) regulating water quality, iv) provisioning habitats, v) provisioning of nurseries and vi) regulating climate. Diverse technological knowledge gaps were identified by both methods at all scales of the macroalgae cultivation process, followed by economic and environmental knowledge gaps depending on the method used. Based on suggestions from the expert respondents in the Delphi process, there is a clear need for an European-wide strategy for reducing risks for seaweed producers, providing clear standards and guidelines for obtaining permits, and providing financial support to improve technological innovation, that will ensure consistent quality. Legal (e.g., safety regulations), economic (e.g., lack of demand for seaweeds in many countries) and technological (e.g., production at large scale) constraints represented almost 70% of the total responses in the Delphi process, whereas environmental and technical constraints were more dominant in the literature. The most commonly identified potential negative impacts of macroalgae cultivation both among the expert responses and the reviewed articles were unknown environmental impacts, e.g. to deep sea, benthic and pelagic ecosystems. The present study provides an assessment of the state of knowledge regarding ES provided by seaweed cultivation and identifies the associated knowledge gaps, constraints and potential negative impacts. One of the main hurdles recognised by the EWG was the understanding of ES themselves by the different stakeholders, as well as the issue of scale. Studies providing clear evidence of ES provided by seaweed cultivation and/or valorisation of these services were lacking in the literature, and some aspects, like cultural impact etc. were missing in the responses to the questionnaires during the Delphi process. The issue of scale and scaling-up was omnipresent both in assessing the ES provided by seaweed cultivation and in identifying knowledge gaps, constraints and potential negative impacts. For example, the ES provided will depend on the scale of cultivation, the main technological knowledge gaps were often related to scale of cultivation. Likewise at a large scale of operations, there could be multiple associated potential side effects, which need to be further investigated. Based on the outcomes of this investigation, we provide an outlook with open questions that need to be answered to support the sustainable scaling-up of seaweed cultivation in Europe.