The Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST) was proposed as an M-class mission in response to the 2010 Call for Medium-size Missions for ESA's Cosmic Vision plan.

The mission concept arose from a consultation process with the scientific community that was carried out in 2009 by the ESA-appointed "Fundamental Physics Roadmap Advisory Team" (FPR-AT).

In February 2011 STE-QUEST was one of four missions recommended by the ESA advisory structure and selected for an assessment study. The assessment phase for STE-QUEST started in April 2011 and will be concluded in mid 2013.

In the meantime, STE-QUEST was studied at the ESA Concurrent Design Facility (CDF) at the European Space Research and Technology Centre (ESTEC) in May 2011. The study concluded with the identification of a preliminary design of the mission and its payload.

The results of the CDF study have been used to prepare an open Invitation to Tender (ITT) to Industry. The ITT process closed in late 2011. Two parallel contracts covering a 1-year assessment study of the STE-QUEST mission are in place. Instrument consortia have been selected through an open call; ESA Member States will financially support instrument-related activities.