If you write EmacsLisp in your LaTeX documents, you might want to have emacs-lisp-mode active when editing the Lisp, and latex-mode when editing the rest of the file. The solution is to use MultipleModes.
Here is a solution using two-mode-mode, by EdwardOConnor:
(require 'two-mode-mode) (setq default-mode (list "LaTeX" latex-mode) second-modes (list (list "Emacs-Lisp" "\\begin{elisp" "\\end{elisp" 'emacs-lisp-mode)) (two-mode-mode)
See LiterateEmacsLisp for a complete example of how to do this.