The Science Programme of the European Space Agency is a long-term programme of space science and space exploration missions. Managed by the agency's Directorate of Science, The programme funds the development, launch, and operation of missions led by European space agencies and institutions through generational campaigns. Horizon 2000, the programme's first campaign, facilitated the development of eight missions between 1985 and 1995 including four "cornerstone missions" – SOHO and Cluster II, XMM-Newton, Rosetta, and Herschel. Horizon 2000 Plus, the programme's second campaign, facilitated the development of Gaia, LISA Pathfinder, and BepiColombo between 1995 and 2005. The programme's current campaign since 2005, Cosmic Vision, has so far funded the development of ten missions including three flagship missions, JUICE, Athena, and LISA. The programme's upcoming fourth campaign, Voyage 2050, is currently being drafted. Collaboration with agencies and institutions outside of Europe occasionally occur in the Science Programme, including a collaboration with NASA on Cassini–Huygens and the CNSA on SMILE.

The Science Programme is managed by the European Space Agency (ESA)'s Directorate of Science, and its goals include the proliferation of Europe's scientific presence in space, fostering technological innovation, and maintaining European space infrastructure such as launch services and spacecraft operations. It is one of ESA's mandatory programmes in which each member state of ESA must participate. Members contribute an amount proportional to their net national product to ensure the long-term financial security of the programme and its missions. The programme's planning structure is a "bottom up" process that allows the European scientific community to control the direction of the programme through advisory bodies. These bodies make recommendations on the programme to the Director General and the Director of Science, and their recommendations are independently reported to ESA's Science Programme Committee (SPC) – the authority over the programme as a whole. The programme's current advisory structure consists the Astronomy Working Group (AWG) and the Solar System and Exploration Working Group (SSEWG), who report to the senior Space Science Advisory Committee (SSAC) that reports to the agency's directors. Membership on the advisory bodies last three years, and the chairs of the AWG and SSEWG are also members of the SSAC. Ad hoc advisory groups may also be created to advise on certain mission proposals or the formulation of planning cycles.

Missions in the programme are selected through competitions in which members of the European scientific community submit proposals to ESA. During each competition, the agency outlines one of four mission categories for which proposals need to meet the criteria of. These are the "L"-class large missions, the "M"-class medium missions, the "S"-class small missions, and the "F"-class fast missions, each with differing budget caps and implementation timelines. The proposals are then reviewed by the AWG, SSEWG, engineers at ESA, and any relevant ad hoc working groups, as part of a feasibility study known as "Phase 0". Missions which require new technologies to be developed are reviewed during these studies at the Concurrent Design Facility at the European Space Research and Technology Centre. After the study, up to three proposals are selected as finalists in "Phase A", in which a preliminary design for each candidate mission is formulated. The SPC then makes a final decision on which proposal proceeds to phases "B" through "F", which include the development, construction, launch, and disposal of the spacecraft used in the mission. During Phase A, each candidate mission is assigned two competing contractors to build their spacecraft, and the contractor for the winning mission is chosen during Phase B.

The European Space Agency (ESA) was established in May 1975 as the merger of the European Space Research Organisation (ESRO) and the European Launcher Development Organisation. In 1970, the governing Launch Programme Advisory Committee (LPAC) of ESRO made a decision not to execute astronomy or planetary missions, which were perceived as beyond the budget and capabilities of the organisation at the time. This meant that cooperation with other government space agencies and institutions was necessary for large-scale scientific missions. This policy was effectively reversed in 1980, when ESA's then-Director of Science, Ernst Trendelenburg, and the agency's new authoritative Science Programme Committee (SPC) selected the Giotto flyby reconnaissance mission to comet Halley and the Hipparcos astrometry mission for launch. In addition to the selection of the International Ultraviolet Explorer observatory in March 1983, the three were the first European science missions launched aboard Arianespace launch vehicles, which gave Europe autonomy over its launch services.

This, in addition to the lack of a long-term plan for scientific missions, along with budget setbacks from NASA on the collaborative International Solar Polar Mission (later named Ulysses), spurred the development of a long-term scientific programme through which ESA could sustainably plan missions independent of other agencies and institutions over lengthier periods. The leadership and advisory structure of ESA's Directorate of Science changed immediately prior to the programme's establishment. In the 1970s, ESA's Science Advisory Committee (SAC), which succeeded the LPAC, advised the Director General on all scientific matters; the Astronomy Working Group (AWG) and the Solar System Working Group (SSWG) also reported directly to the Director General. In the early 1980s, the SAC was replaced with the Space Science Advisory Committee (SSAC), who were tasked to report to the Director of Science on developments in the AWG and SSWG. In addition, former SAC chair Roger-Maurice Bonnet replaced Trendelenburg as Director of Science in May 1983.

In November and December 1983, ESA made the first open call for mission proposals to the European scientific community, based on an idea for a community-driven programme presented by Bonnet to the SPC in late 1983. The call yielded 68 proposals – 30 in the field of astronomy and 34 in the field of solar physics, with 4 miscellaneous concepts also submitted. An ad hoc "survey committee" led by then-SRON Director Johan Bleeker was convened, consisting members of the SSAC, CERN, the European Science Foundation, the European Southern Observatory, and the International Astronomical Union, to examine the proposals submitted. Throughout early 1984, the survey committee formulated plans for a series of missions divided into three categories – "cornerstones" which would cost two annual budgets over a long implementation timeline, medium-size missions which would cost one annual budget, and small-size missions that would cost half an annual budget.

"Over the past 25 years, space science has progressed from the pioneering and exploratory stage to a firmly established mature branch of fundamental science. The time has come to identify what the main thrusts of European space science should be for the coming decades to consolidate Europe's position in the forefront of scientific development."

The budget for the Science Programme was 130 million accounting units (MAU) annually in 1984, and a 7% annual increase until 1991, when the budget would be fixed at 200 MAU per year onwards, was proposed. Medium-size and small-size categories would later be merged into a single medium-size category that would represent missions costing half a budget. This category was internally referred to as the "blue missions", named after their representation as blue boxes in a publicised diagram of the plan. Each of the original three cornerstones of the plan were assigned a specific field of science that competing proposals would aim to fill, while the objectives of medium-sized missions were left open to be competitively selected alongside mission proposals. The cornerstones selected were a comet sample-return mission, an X-ray spectroscopy mission, and a submillimetre astronomy mission. Cornerstone objectives that were not selected due to financial and technical shortcomings, but mentioned by the survey committee as possibilities beyond Horizon 2000, included a solar probe, a Mars rover, and a two-dimensional interferometry mission.