The local currency in Peru is the Nuevo Sol S/. ( Sol means sun).

You can exchange USA dollars or Euros at almost any place, banks, money changing offices, hotels, restaurants and with street money changers. Beware of false notes. You are better off exchanging at money changing offices or banks. It  is  better to obtain your Soles in Peru than in your home country as you will obtain better exchange rates but beware of fraudulent notes and unofficial money traders offering you lucrative deals on the streets.

You can buy soles in the UK at around 4.3 to the Pound (source: compareholidaymoney.com) and in the USA at around 3.1 soles to the Dollar (source: xe.com) as of April 2015. The exchange rate in Lima in October 2016 is at around 3.3 to 3.4 soles per 1 US$

Many ATMs are available for all major credit cards (Visa, Master Card, Cirrus, etc.), but you need to take cash if you go to Aguas Calientes. ATMs and banks are a rare find in those areas. Most ATM machines in Peru charge for withdrawing money from your home bank accounts. Presently the only Bank that adds no surcharge is the Scotiabank. At the Lima airport most ATMs are Globalnet, but after arrivals come out and take the stairs on your left (or lift just after the stairs) once at the top take a right and then after the Serpost post office on the left there is a cafe and opposite there are a bank of ATMs, Scotiabank, BCP, and BBVA. If going from departures just turn left, past Starbucks and then on the right. You can withdraw USA dollars or Peru Nuevos Soles.ATMs offer English and Spanish instructions.

If you want to buy tours in Cusco like Machu Picchu or Manu trips, you get a much better total cost paying the tour operator in cash. But that means you need a lot of cash from ATM’s. Tell your bank in advance, that you will be withdrawing substantial amounts of cash, so they can raise your limit. Many ATM’s are limited to how much you can withdraw per transaction, often 300USD or 700Soles. 

You can redeem travellers checks at most banks, but you will get lower exchange rates and be charged a comission. Do not  use travellers checks in Peru, as many places do not accept them. Only the owner of the traveller check can cash it at a bank, no endorsments are allowed.

The USA dollar is accepted at almost all comercial places, and many times, prices are quoted both in USA $ and S/. You can pay with dollars and receive dollars or soles in exchange. Bring new crispy low denomination bills as they are well accepted by money changers. Old, torn bills are not accepted. Like in the USA, any transaction in cash for USA$10,000 or more has to be reported to the local authorities.

Most U.S. banks will change USA dollars to soles with a few days notice and a minimal fee, thereby not having to deal with exchanging in Peru, but will give you a much lower exchange rate.