Sudoku - Two Questions

[emanymton]

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Sudoku is a game requiring the use of logic, reasoning, thinking and ‘trial and error’ to solve. The games are rated up to and including fiendish and insane (where a low number of squares are given).

Two questions:

1. Is it possible to solve challenging Sudoku games using only reason and logic (i.e. without guessing and 'trial and error)?

2. Is there a limit of ‘givens’ where the Sudoku game becomes unsolvable? I have worked out a ‘23 given’ Sudoku game.
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Updated to correct spelling errors.

[Nikia]

1. I have played Sudoku games where I only used reason and logic

Including ones in the daily paper. I have been able to find at least one square in which I can figure out the number without guessing. The more squares you fill in, the more likely that you will find a square which only has one logical answer.
The more squares that are not given from the start, the harder it is and less likely that you will find a square which only logically has one number possibility.

[ironflange]

2. If they're properly constructed, they can be solved without guessing

That said, there are some pretty arcane strategies that you have to use to solve some of the bastards. These are pretty well beyond my capabilities; I decided a while ago to stick to crosswords.