‘Marx on tax’ as an effective antidote to inequality is an overlooked theme within his own output, but also for our own time. Marx theorising on tax is seen even by pre-eminent Marxists as an empty box, but Marx and Engels in fact had plenty to say about tax. Their coverage embraces progressive taxes, both on capital and income, a strong preference for direct over indirect taxation, inheritance tax, land-value tax, taxes on financial transactions, and state finances around the world. Tax also provides the battleground for a rare sight of Marx as campaigning activist, in 1848, matched in the same period by close ally Wilhelm Wolff. The tax policies of Marx and Engels have been neglected because they are primarily to be found in their journalism and letters. They are no anachronistic curiosity but perfectly applicable to the income and wealth inequalities of our own era.