If you are already proficient in solving Sudoku or other logic and thought puzzles then a "Difficult" Sudoku is exactly right for you. It is quite demanding and if it is still not difficult enough, try a "very difficult" level.
Try to fill 9 blocks with numbers from one to nine so that each digit occurs only once per row, column, and block.
You can find useful background knowledge, rules and tips on Sudoku on our homepage. In addition, you can print Sudoku's in all difficulty levels (including solutions).