Personal income tax in Poland has a progressive structure. Depending on your income you will be charged 19%, 30% or 40% of income tax, depending on the income.

There are three income thresholds (tax scale for year 2008):

• 0 - 44 490 PLN - 19% minus tax reducing amount of 586.85 PLN
• 44 490 - 85 528 PLN -  6,504.48 PLN + 30% of the surplus over 44 490 PLN
• 85 528 PLN and more - 20 177,65 PLN + 40% of the surplus over 85 528 PLN

Polish personal income tax system is generally very complicated. Apart from the progressive tax scale there are many additional exemptions and allowances. To minimize the total amount payable to the tax system, you might considering a tax advisor consultation.

Foreign nationals working in Poland must register for tax and obtain an NIP number (tax identification number). Depending on the total amount of time spent in Poland during a tax year, foreigners can have limited or unlimited tax liability.

• a limited tax liability - applying to those staying in a country for less then 183 days; taxed on the income generated only in Poland; Additionally some types of income may be taxed at a flat rate of 20%.
• Unlimited tax liability - applying to permanent residents and expats who are staying in Poland for more than 183 days a tax year; will be taxed on the income from all sources worldwide

During the tax year the taxpayers are obliged to make advance payments for their income tax (by the 20th day of the following month for the preceding month) and, after the end of the given tax year, pay the tax due in a final amount (i.e., not later than April 30 of the following year).

Sources :
http://www.paiz.gov.pl/
http://pl.wikipedia.org/