Connect jquery + fix $('nav__nav') to $('.nav__nav') (point lost)!
$('#nav__btn').click(function() { $('.nav__nav').slideToggle('slow'); });
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.0.1/jquery.min.js"></script> <div> <ul class="nav__nav"> <li class="nav__item nav__active"> <a class="nav__home" href="#"> <span>Home</span> </a> </li> <li class="nav__item"> <a class="nav__website" href="#"> <span>Isuzu Website</span> </a> </li> <li class="nav__item"> <a class="nav__isis" href="#"> <span>ISIS</span> </a> </li> <li class="nav__item"> <a class="nav__power" href="#"> <span>Truck Power</span> </a> </li> <li class="nav__item"> <a class="nav__sheets" href="#"> <span>Spec Sheets</span> </a> </li> <li class="nav__item"> <a class="nav__images" href="#"> <span>Images</span> </a> </li> <li class="nav__item"> <a class="nav__video" href="#"> <span>Video</span> </a> </li> <li class="nav__item"> <a class="nav__survey" href="#"> <span>Online Survey</span> </a> </li> <li class="nav__item"> <a class="nav__blog" href="#"> <span>Blog</span> </a> </li> <li class="nav__item"> <a class="nav__contact" href="#"> <span>Contact</span> </a> </li> </ul> <a href="#" class="nav__btn" id="nav__btn">Menu</a> </div>