This has happened because the Republicans and Democrats could not to come together and pass a bill funding government services into October and beyond.
The Republicans control both chambers of Congress, but in the Senate – or upper chamber – they are short of the 60 votes they need to pass a spending bill.
Opposition Democrats, therefore, have some leverage.
Both sides have been in a bitter standoff over healthcare spending, and the Democrats have refused to back a Republican bill that they say will make it harder for Americans to afford healthcare.
They are calling for an extension of tax credits – which are set to expire – that make health insurance cheaper for millions of Americans, and for a reversal of cuts to Medicaid that have been made by Trump. Democrats also oppose spending cuts to government health agencies.
A stopgap bill was earlier passed in the House, or lower chamber, but is yet to clear the Senate.
And so, at 00:01 EDT on Wednesday (04:01 GMT), it became official: the US had its first shutdown for nearly seven years.