How would I use a navigation menu over multiple pages?

I’m building a website that’s going to have hundreds of pages. I’m looking for a way to include a navigation menu from one file, once, so when the menu changes I only have to change one file to change the menu on every page instead of having to make the change in every single file. I would normally use iframes for this making one frame the menu and the other the content pages. How can we do this without using iframes?