How do I control Emacs's case-sensitivity when searching/replacing?

For searching, the value of the variable case-fold-search determines whether they are case sensitive:

  (setq case-fold-search nil) ; make searches case sensitive
  (setq case-fold-search t)   ; make searches case insensitive

Similarly, for replacing the variable case-replace determines whether replacements preserve case.

To change the case sensitivity just for one major mode, use the major mode's hook. For example:

  (setq XXX-mode-hook
        (function
         (lambda ()
           (setq case-fold-search nil))))