As part of a broad horizon scan, the ESA Earth Observation Science Strategy Foundation Study, combined with user consultation, consolidated a set of 23 non-exclusive guiding scientific questions in support of the Earth Observation Science Strategy. These questions, which contribute directly to the priority science themes identified in the ESA Earth Observation Science Strategy, provide a framework for establishing activities within ESA’s Earth Observation Programmes. This version of the science questions is the result of extensive consultations with the Earth observation user community and stakeholders in the Earth Observation Programmes. These include: The Advisory Committee on Earth Observation (ACEO) composed of independent scientists advising the ESA Director of Earth Observation Programmes Two User Consultation workshops in 2023 and 2024 with open participation followed by online questionnaires ESA Member States and associated national delegations The resulting science questions focus on pressing Earth system science issues that can be addressed by using Earth observation (in combination with other datasets) This report represents a key element of the final output from the Science Strategy Foundation Study, and summarises the guiding science questions and the process followed in their derivation. Each science question is expressed as a brief narrative, and a summary table of the main characteristics and geophysical observable requirements.