The state of Wisconsin has the oldest continuously operating state income tax in the United States. In this brief I discuss some of the history and background of the state income tax, as well as the current distribution of taxes and effective tax rates. The state has a progressive income tax system, with statutory marginal tax rates that increase with income. In recent years, the state income tax has accounted for about 45% of Wisconsin’s state tax revenue, and amounted to around 3% of total personal income. In 2019, over 3.1 million taxpayers filed state returns, paying on average $2,639 in state income taxes, at average effective rate of 4.32%. Across the distribution, average effective tax rates increased from 1.3% to 5.6%. The progressive nature of the tax system, along with the skewed distribution of income, means that most state income taxes are paid by high income filers. In 2019, the bottom 40% of filers, with incomes less than $26,600 paid only 1.5% of all state income taxes, while the top 20% of taxpayers, who had incomes over $85,330, accounted 72.6% of income tax revenue. Accounting for exemptions, deductions, and tax credits, the filers that accounted for the vast majority of all income tax revenue paid effective marginal tax rates around 5.8%.