Does an underscore “ _ “ and hyphen affect SEO?

[Hal Smith]

Are you talking about their use in SEO URLs? If so then use hyphens as Google treats a hyphen as a word separator, but treats an underscore as a word joiner. For example, my-own-blog is "my own blog" to Google, and can be optimized as such for "my own blog" keyword. But "my_own_blog" is the same as "myownblog" and I doubt it that you'd be optimizing your site for such words. Google has confirmed this many times themselves that you should use hyphens or dashes, as put by Google's very own 'Matt Cutts', is the way to go.

Hal Smith
URLdreamer Consultant