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As with Sudoku, the objective of Killer Sudoku is to fill the grid with the numbers 1 to 9, such that each row, column and nonet (3x3 group of cells) contains each number only once.

In addition to this, a Killer Sudoku grid is divided into cages, shown with dashed lines. The sum of the numbers in a cage must equal the small number in its top-left corner.

The same number cannot appear in a cage more than once.

Greater-Than Killer Sudoku, or Comparison Killer Sudoku, has exactly the same rules as Killer Sudoku, except that not every cage has a sum in its top-left corner. Instead, some cages are linked together with symbols:

• The left cage's sum is less than the right cage's sum.
• The left cage's sum is less than or equal to the right cage's sum.
• The left cage's sum is equal to the right cage's sum.
• The left cage's sum is greater than or equal to the right cage's sum.
• The left cage's sum is greater than the right cage's sum.

The Strategies page describes strategies for solving Killer Sudoku puzzles.