You will be able to travel to the USA with an Electronic System Travel Authorisation (ESTA) instead of a visa if you're a citizen of a country such as the UK, covered by the Visa Waiver Programme. You should apply for your ESTA at least 72 hours before your scheduled departure. If you fail to obtain an ESTA then you won’t be allowed to travel. You will also require a valid e-Passport for using ESTA. In fact, ESTAs are valid for multiple journeys for up to two (2) years or until your passport expires, whichever is sooner.