Poland can be easily accessed by plane, train, bus, car and even by boat.
Warsaw Chopin Airport, Katowice International Airport, John Paul II International Airport are the major airports of Poland. There are other airports at Gdansk and Rzeszow. A lot of the major European airlines fly to Poland's airports, with many having direct connections. Flights also come in from Canada, China and the United States. These fly in non-stop from Toronto, Beijing, New York and Chicago. Most international airlines fly into the Warsaw airport. LOT Polish Airlines is the national carrier of this country.
Several international bus lines offer their services to drive into Poland, from important European cities. The most prominent bus service providers are Eurolines, Ecolines, Lux Express and Tiger Express.
PKP Intercity is the long-distance train service that is available here. It connects you directly to Warsaw from Oberhausen, Berlin, Vienna, Kiev, Prague, Moscow and Budapest.
Poland is a part of the Schengen area, so there aren't any checkpoints anymore. You can easily drive in from any of the major roads linking Poland to its neighbours. The non-EU countries such as Belarus, Russia and Ukraine have long queues, so if you are driving in from those areas, be prepared to wait longer. US citizens who are driving in will need to carry an international license since Polish authorities require it.
Boats come in from Sweden, Denmark and even Germany. Stops include Copenhagen, Rostock and Ystad. Gdansk, Hel, Gdynia have a lot of large marinas where yachts and other ferries alight.