Killer Sudoku (House Of Puzzles)

Killer Sudoku is the most challenging of the Sudoku variants. Some see it as the most pure form of number/logic puzzling. You start with no numbers filled in on the grid and you must use logic and simple addition to fill in all the numbers. The original rules of Sudoku apply: each column, row and 9x9 box must contain the numbers 1 to 9 once only.