This book contains the first collection of 50 handcrafted Challenging Sudoku Variations, arranged in the difficulty levels Beginner (18 puzzles), Advanced (16 puzzles) and Professional (16 puzzles), with grid sizes ranging from 6 by 6 up and until 12 by 12. This book is a must have for all Sudoku enthusiasts who want to tackle other than standard Sudokus and who dare to tease their brain to the utmost to solving these challenging Sudoku Variations, appealing to various kinds of logic to find the clues for concluding the unique solution. You will be challenged with Sudoku Variations with one or more extra groups of coherent cells with nice patterns and figures that also have to contain the different digits exactly once. You even will be challenged more by the Sudokus with extra groups of coherent cells with special features (Dokus). To solve these Sudoku Variations you literally have to think "out-of-the-box" for these exiting new challenges. Use your math skills at the CalculoDoku and SeriesDoku, Sharpen your two-dimensional insights at the InfinityDoku, and appeal to your spatial expertise at the JigSawPiecesDoku, PalindromeDoku and SpiraloDoku. Enjoy the Word Sudoku variations with different letters for finding meaningful words as extra solution of these puzzles. Some Sudokus are combinations of various types of logic. The answers are included at the end. Give yourself a well-earned pat on your shoulder when you have cracked the features of these Sudokus after many hours of brain entertaining puzzling fun, while sharpening your concentration and solving skills.