Minimum number of clues for a sudoku puzzle

In a sudoku puzzle, what is the absolute minimum number of clues that must be given to specify a layout with a unique solution? Is one in each row, column, and box (9 total) necessary or sufficient?

Answer

For a standard $9 \times 9$ sudoku, the minimum is $17$ squares. It had long been known that at least $16$ were required and that $17$ was sufficient. This article closes the gap.