<p>I am writing a text editor for Mac OS X.  I need to display hidden characters in an NSTextView (such as spaces, tabs, and special characters).  I have spent a lot of time searching for how to do this but so far I have not found an answer.  If anyone could point me in the right direction I would be grateful.</p>

Have a look at the NSLayoutManager class. Your NSTextView will have a layout manager associated with it, and the layout manager is responsible for associating a character (space, tab, etc.) with a glyph (the image of that character drawn on the screen).
In your case, you would probably be most interested in the
```
replaceGlyphAtIndex:withGlyph:
```
method, which would allow you to replace individual glyphs.