The US Dollar

The legal currency in US is the United States dollar (USD or US$ or $).

Money Exchange (Cash)

Usually airport, and hotels has worst rates and charges higher or in some cases exorbitant commission, so change only minimal required currency at these kinds of places.
Seattle downtown has currency exchange counters like Travelex currency Services and others, however rates are usually little bit higher than banks.
You can also exchange currency at banks, however will have to pay fees/commission. If you are changing large amount of currency then it is worth checking few banks as every bank exchange rates and fees differs slightly.
You can also buy US Dollars at home from your bank before getting on to the plane for Seattle.

Credit cards

Your credit card provides you with absolute best exchange rate for purchases in US. Please check beforehand with your bank for the additional fees on the top of exchange rate for using your card in foreign country before you decide to use you credit card. Visa and Master cards are widely accepted all over in the Greater Seattle area establishments.

ATM

ATM machines are all over in Seattle allows cash withdrawal in local US$ using international credits cards like Visa/Plus and Mastercard/Cirrus. Please check with your bank for any additional fees for using your ATM card in foreign country before you decide to use you ATM card.

Traveller cheques

Traveller's cheques are accepted by hotels and banks in US. Traveller cheques may be subject to a fee.