Before acting as pilot in command of a complex airplane, 14 CFR 61.31(f) requires the same two-part path as the high-performance rule:
Like the high-performance endorsement, it is one-time and not type-specific — once earned, it covers all complex airplanes you are otherwise qualified to fly.
Pilots who logged PIC time in a complex airplane before August 4, 1997 do not need the endorsement.
For a complex seaplane, the controllable-pitch element is satisfied by an airplane with an adjustable, feathering, or constant-speed propeller. The retractable-gear element is met by retractable water rudders or amphibious gear as applicable to the design.
For years the FAA required the commercial single-engine practical test to be conducted in a complex airplane. In 2018 that requirement was removed — the commercial ASEL checkride may now be flown in a technically advanced airplane (TAA) or a complex airplane. This does not change 61.31(f): you still need the complex endorsement to act as PIC of a complex airplane regardless of how you took your commercial checkride.
Most training airplanes used for the complex endorsement are the Piper Arrow, Cessna 172RG/182RG, Mooney M20, or Beechcraft Sierra/Bonanza — each has the retract/flaps/CS-prop trio.
The complex endorsement is a one-time, instructor-issued logbook sign-off — no examiner, no written test. Earn it once and it covers every complex airplane you are otherwise rated to fly, though good judgment still calls for transition training in any unfamiliar make and model.