For several months now you have been able to pay using a Contactless Credit or Debit card on London’s buses. From tomorrow, contactless payments will be extended to Tube, tram, DLR, London Overground and most National Rail services in London.
Check whether your debit/credit card supports this technology by looking for this symbol on the face of the card:
Be aware that some pre-paid debit cards are now accepted, but most UK issued cards are usable. Internationally, this is a helpful guide produced by TfL: