The most convenient "gateways" to the Pacific Northwest are Seattle and Portland. Both of these cities are well-connected to the rest of the world by plane, car and train, and are good starting points for trips to other areas of the Pacific Northwest.
By plane
The main airports in the Pacific Northwest are Seattle-Tacoma International Airport and Portland International Airport. Both airports offer flights to most cities in the USA and some flights to Asia and Europe. Other smaller airports are sprinkled around the Northwest, but most have few commercial flights except to other places in the Pacific Northwest.
By train
Amtrak offers three routes serving the Northwest.
Amtrak Cascades: Runs from Eugene to Vancouver, BC with major stops in Seattle and Portland.
The Coast Starlight: This scenic route connects Seattle and Portland to California: this route is often hours late, so for regional trips the Amtrak Cascades is more reliable.
The Empire Builder: Starts in Chicago and goes west to Spokane, where it splits up into two branches: one goes to Portland and the other goes to Seattle.
By car
I-5 runs north-south from the Canadian border through Seattle and Portland to California. From the east, there are two main options: I90, from Seattle east to Chicago and Boston, and I84 which starts in Portland and goes southeast to Salt Lake City.