In the European union visas are not needed.

Citizens of the following countries that can travel to Poland without Visas:

Andorra, Argentina, Australia, Austria, Belgium, Brazil, Brunei, Bulgaria, Canada, Chile, Costa Rica, Croatia, Cyprus, the Czech Republic, Denmark, El Salvador, Estonia, Finland, France, Germany, Greece, Guatemala, Honduras, Hong Kong (special administrative region), Hungary, Iceland, Ireland, Israel, Italy, Japan, Latvia, Liechtenstein, Lithuania, Luxembourg, Macao (special administrative region), Malaysia, Malta, Mexico, Monaco, the Netherlands, New Zealand, Nicaragua, Norway, Panama, Paraguay, Portugal, Romania, San Marino, Singapore, Slovakia, Slovenia, South Korea, Spain, Sweden, Switzerland, the United Kingdom, the United States of America, Uruguay, the Vatican, and Venezuela.
More info about Visa requirements

Visas are required for the citizens of Belarus, Ukraine and Russia.

In the end of 2007 Poland joined The Shengen territory. Within territory of this agreement there is no passport control. All the members of European Union belong to Shengen except for two: United Kingdom and Ireland