The rules of Killer Sudoku are straightforward:
place a digit from 1-9 in each empty cell so that each row, each column and each 3x3 block contains all the digits from 1-9;
the digits in each inner shape, or cluster, must add up to the number in the top-left corner of the cluster;
no digit can be repeated in a cluster.
See the Puzzler Encyclopedia for a fuller explanation and step-by-step instructions. To benefit from the full range of interactive options, take a moment to read the Help file in the puzzle window.