I can’t believe I’m writing an article about something so simple yet I’ve been asked this countless times. Does using an underline or hyphen effect how search engines index your content therefore affecting your search viability? Yes, the secret to ranking high is replacing your underlines with hyphens (kidding).
What has Google said about using underline vs hyphens:
Google’s webmaster guidelines “recommend that you use hyphens (-) instead of underscores (_) in your URLs.”
Most recently, Matt Cutts explained the technical reasoning behind having hyphens as a separator.
In summary he concludes that this has to do with how Google’s links search term words and that they see that words with a hyphen (-) belong together, whereas underlining does not. However, he also points out that if you currently have a website online which uses underlines than stick to that as it is not worth the effort in changing it. (See video for full explanation)