People have been shivering in temperatures as low as -52 degrees Celsius in Canada's Quebec City. Ottawa and Montreal were also badly affected.
New Jersey, New York, Connecticut and Massachusetts were among the US states hit by Arctic conditions. New York city was covered in a blanket of snow.
Schools were closed, doors froze to their frames and flights were shelved.
New York mayor Michael Bloomberg was particularly concerned for homeless people.
"City shelters are well prepared," he said. "Everyone wanting and needing a bed will get one."
Disaster
Another US city, Boston, was suffering its coldest temperatures for 20 years.
One official described the weather as "a natural disaster".
There is some good news, though.
The cold snap isn't expected to last and things should be back to normal in many places by the start of next week.
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