Stansted Express
47 minutes from London Liverpool Street to Stansted Airport. Trains every 15 minutes (every 30 minutes overnight). ~£20–24 single online; cheaper if booked in advance.
National Express A6 / A7 / A9 coaches
Direct from Liverpool Street, Stratford, Victoria, Golders Green and Marble Arch. ~1h 15m–1h 45m. From £6 booked ahead. Operates 24/7 — useful for very early or very late flights when the Tube is closed.
Terravision / Airport Bus Express
Similar coach service from Victoria and Liverpool Street; typically a few pounds cheaper than National Express.
Taxi / private hire
£80–£130. Around 60–90 minutes on the M11.
Tip
Stansted has only one terminal — no inter-terminal transfer needed. The express train station is directly under the terminal building.