TRL 1Basic principles observed and reported.
TRL 2Technology concept and/or application formulated.
TRL 3Analytical and experimental critical function and/or characteristic proof-of-concept.
TRL 4Technology basic validation in a laboratory environment.
TRL 5Technology basic validation in a relevant environment.
TRL 6Technology model or prototype demonstration in a relevant environment.
TRL 7Technology prototype demonstration in an operational environment.
TRL 8Actual Technology completed and qualified through test and demonstration.
TRL 9Actual Technology qualified through successful mission operations.

NASA developed Technology Readiness Levels (TRLs) in the early 70’s as a means of assessing whether emerging technology was suitable for space exploration. By the 1990’s it was in use across many US Government agencies, including the Department of Defense (DOD) and Department of Energy (DOE). TRLs are now in common use in the UK.