This report documents the program and the outcomes of Dagstuhl Seminar 12181 "Quality of Experience: From User Perception to Instrumental Metrics". As follow-up of the Dagstuhl Seminar 09192 "From Quality of Service to Quality of Experience", it focused on the further development of an agreed definition of the term Quality of Experience (QoE) in collaboration with the COST Action IC1003 "Qualinet", as well as inventories of possibilities to measure QoE (beyond the usual user polls) and to exploit feedback between users and systems that reflects QoE issues. The report furthermore describes the mode of work throughout the seminar, with focus on personal statements by the participants, results of the group works, and open challenges.