The Living Planet Programme of the European Space Agency encompasses a science-driven strategy for monitoring the Earth from space. The Earth Explorer missions are defined, developed and operated in close cooperation with the science community and focus on the key components of the Earth System: the atmosphere, biosphere, hydrosphere, cryosphere and the Earth's interior. The emphasis of these missions is on providing data to advance our understanding of these individual components, their interaction with each other and the impacts that human activities have on natural Earth processes. By involving the science community from the beginning and introducing a peer-reviewed selection process, this ongoing user-driven approach has given the Earth science community an efficient tool in their endeavor to better understand and monitor our planet. So far, this process has resulted in six missions currently under development: GOCE, Cryosat, ADM Aeolus, SMOS, Swarm and EarthCARE. The third cycle of Earth Explorers Core Missions was started in 2005 to select the seventh Earth Explorer mission due to launch in 2014/2015. At present there are six candidate missions being assessed at pre-phase A level. These missions were chosen to enter the assessment phase as a result of the Call for Core Mission ideas released by ESA in 2005, which resulted in 24 proposals out of which six candidates were selected: - BIOMASS - global measurements of forest biomass and extent; - TRAQ - (TRopospheric composition and Air Quality) - Air quality monitoring and long-range transport of air pollutants; - PREMIER - (PRocess Exploration through Measurements of Infrared and millimetre-wave Emitted Radiation) Understanding the processes that link trace gases, radiation, chemistry and climate in the atmosphere; - FLEX - (FLuorescence EXplorer) - Observation of global photosynthesis through the measurement of fluorescence; - A-SCOPE - (Advanced Space Carbon and Climate Observation of Planet Earth) - Improving the understanding of the global carbon cycle and regional carbon dioxide fluxes; - CoReH2O - (Cold Regions Hydrology High-resolution Observatory) - Detailed observations of key snow, ice and water cycle characteristics. This paper presents an overview of the six candidate missions, describing the scientific objectives and outlining the main aspects of the candidate implementation concept currently under evaluation.