An understanding of how Oyster cards work is essential if you're planning to use public transport at all during your visit to London.
An Oyster card is an electronic 'smartcard' the size of a credit card that has now largely replaced traditional paper tickets.
It really is the most cost-effective and convenient way of getting around London. Also, being a 'smartcard', it does most of your thinking for you, meaning that you don't have to worry about paying the correct fare for the zones you're travelling through, or whether it's the 'peak' or 'off-peak' period.
At the end of every journey, Oyster simply deducts the correct fare.
Although it is still possible to get around London without using an Oyster card, the cost of buying an old-fashioned single paper ticket is now so much higher (almost double) that it really isn't worth considering.
Click here to visit Transport for London's website and get a summary of the most up-to-date fares.
Once you've arrived in London, the easiest way to get hold of an Oyster card is to go to one of the touchscreen ticket machines at any London Underground station.
If you speak a language other than English, touch the blue bar at the bottom of the screen to display a list of the languages available and pick the one you want.
Then touch the box at the bottom-right-hand corner of the screen that says 'Buy Oyster Card'. There's a small refundable deposit for each card (you'll need one per person), and you will also be asked to select the amount of prepaid credit you wish to load onto the card - £5, £10, £30, whatever you like.
Insert cash or pay with a card, and that's it! The machine will spew out your Oyster card, loaded with credit, and you'll be ready to start using it.
Whenever you find yourself running low on credit, simply top up your balance at one of the ticket machines by touching your card to the yellow reader and following the onscreen instructions.
At the end of your stay, you can return your Oyster card to a London Underground ticket office for a refund of the deposit and any unused credit.
If you're planning to return at any point in the future, however, it makes sense to keep hold of your Oyster card. Neither the card nor the credit loaded onto it will ever expire, so it's possible to keep reusing it again and again, trip after trip.
If you're visiting from abroad, or from elsewhere in the UK, you can order a Visitor Oyster Card in advance of your trip from the Transport for London online shop. These can be preloaded with pay-as-you-go credit, to which will also be added a small activation fee. And then there's the cost of postage, which will vary according to where you live.
This has its advantages, of course, the main one being that your card will be in your possession when you arrive, ready to use. If and when you run out of credit, all you have to do is top up your balance using one of the ticket machines at any London Underground station, just as you would if you'd obtained your Oyster card on arriving in London.
On tube, overground, DLR and National Rail services within London, all you need to do is touch in and out at the beginning and end of your journey by pressing your Oyster card to the yellow card reader.
When you place your Oyster card flat against the card reader, a green light, accompanied by one beep, means that the card has been accepted for travel. A red light, accompanied by two beeps, means that it has been rejected. Your card may be rejected because there isn't enough credit on it, in which case you will need to top up before continuing your journey.
However, the readers can be temperamental, so, if your card won't initially let you through the barriers, try again a few more times before going to top up or seeking assistance. Nine times out of ten, it will work on the second or third attempt.
If you're at a row of ticket gates, it's also worth trying a different gate, as this can sometimes do the trick.
In most cases, it's virtually impossible to forget to touch in and out as the automatic ticket barriers at the entrances and exits of plaform areas won't open until you have touched your card to the reader.
Sometimes, however, there are no ticket barriers, in which case freestanding Oyster readers like the one pictured on the right will have been installed, and these can be easy to miss (see the DLR page for a word of warning on this).
It's VERY IMPORTANT that you remember to touch both IN and OUT when using Oyster pay as you go.
If you don't do this, you will be charged the maximum Oyster fare for an 'incomplete' journey.
Some pitfalls to avoid...
NB: On buses and trams, where flat fares apply whatever the length of your journey, you only have to touch in when you board. It isn't necessary to touch out again.
With riverboat services, there are no automatic readers and you will need to present your Oyster card at the ticket kiosk before you travel.
Press your card flat to the yellow reader at any touchscreen ticket machine and it will automatically flag up how much credit you have left.
Electronic displays as you pass through ticket barriers and/or touch your card to the freestanding readers are also supposed to tell you how much credit is remaining on your card, but sometimes these don't work.
In addition to pay-as-you-go credit, you can also load season-ticket Travelcards (flat-fare passes that can be used as many times as you like during a set time period) and bus passes onto your Oyster card.
For more information on how these work, see the page on London Travelcards.