Esta (Electronic System for Travel Authorisation) is part of the USA's visa waiver programme and you'll need to apply if you're heading to the States.
If you attempt to travel to the States without having applied or after being rejected, your travel insurance almost certainly won't cover the cost of your wasted flights and accommodation
If you're travelling to the USA, you'll need to get permission from Esta first.
Esta is the Electronic System for Travel Authorisation and travellers can apply directly via a US Homeland Security Esta website.
Esta is part of the Visa Waiver Programme, which allows visitors from certain countries, including the UK, to travel to the United States for business or pleasure trips of 90 days or less without the need to obtain a visa.
It was introduced to modernise the old Visa Waiver form, which had the drawback of allowing visitors to arrive in the States before finding out whether or not they were eligible to enter.
Under Esta, would-be visitors can now check in advance whether they are eligible to travel or not, reducing the risk of wasted plane fares and other bookings.
At the time of writing (June 2013) the fee to apply was $14 per person, which included a $4 administration fee, and Esta only accepted Mastercard, Visa, American Express and Discover credit cards.
A form on the website asked for details including name, passport number, flight details and where the traveller would be staying in the US.
The applicant was also required to declare any communicable diseases and whether or not they had a criminal record.
You can apply individually or as part of a group of applications for two or more people, such as your family.
Reference numbers are not emailed to applicants so it is advisable to print yours off or write it down, although there is a section on the website where you can retrieve previously submitted authorisation.
Many applicants find out within a few seconds whether their application has been successful, although in some cases people do have to wait for up to 72 hours.
If your application to travel is rejected you will still have to pay the $4 administration fee.
Be wary of the many firms claiming online that they can make the application on your behalf
There is no point applying again, as the website states a reapplication will also be turned down.
If your application is turned down and you still wish to travel, you can apply for a non-immigrant visa at a US Embassy or Consulate.
You can only apply for Esta online, although friends, relatives and travel agents are allowed to fill in the form on your behalf.
Be wary, though, of the many firms claiming online that they can make the application on your behalf.
They will charge you extra for it and you can't be sure they will do what they say they will. Follow the link above to make sure you are clicking through to the legitimate Esta website.
An Esta also allows travel to Mexico, Canada and the Caribbean and is valid for two years, unless your passport expires in that time and you have to renew it.
Travellers should note that even if their Esta application is successful, there is no guarantee they won't be quizzed by customs officials and potentially turned away from the States - it simply means that they are 'eligible to travel'.
However, this is very rare so long as you have not lied on the form about things such as criminal records!
Going through the Esta process is important because if you attempt to travel to the States without having applied or after being rejected, your travel insurance almost certainly won't cover the cost of your wasted flights and accommodation.