What does not work? Do you get a PHP error?

A blank page for exmaple might mean that there's a fatal error, because the .html file cannot get found by PHP, but PHP is configured to not display errors.

If header.html is in the same directory as _main.php, where I guess you tried the code, you might want to try

<?php require( dirname(__FILE__).'/header.html' ); ?>

Also

error_reporting( E_ALL );
ini_set( 'display_errors', 'On' );

might help.

I figured this out too.. simply make sure your header is a .php file and is in the skins directory of the skin you're using. Also make sure to call to the .css file in the main.php file.

I tend to figure things out a few days after posting them ... *sometimes

I just installed b2evolution and am in the process of creating an html header. Your previous experience could possibly help save much time and frustration. Do you mind posting or emailing me the css, php and html code you used for reference? I am creating a header with menu buttons, etc.

Thanks in advance for the help.