Q: Do I need an agent to get published by a traditional publishing house?
A: Depends on how you define “traditional publishing house.”
If you want to be published by a major publisher (the “Big Five” in New York: The Penguin/Random Group, Hachette, Simon and Schuster, HarperCollins & Macmillan; or other large independent houses such as WW Norton, Perseus, Chronicle Books and more) yes, you must have an agent. A way to judge: go to your local bookstore (a real one, not a virtual one.) Most publishers with many books sold through bookstores required agented submissions. (Don’t look on the spines, by the way—look on the copyright page. Each of the publishers I mention above is a conglomeration of many “imprints” that each once were independently owned publishing companies. So you’ll see “Crown” or Knopf or Doubleday or Bantam or Viking on a spine—they’re all now Penguin/Random.)
But “traditional houses” include any company that is selective in its output and doesn’t expect money from you to publish your book (or doesn’t expect you to fund the publication in some other sneaky way, like contracting to purchase 500 copies.)
University presses, small literary presses, specialty publishers all would be considered “traditional,” but may be willing to review your unagented submission. How to tell? Don’t query blindly, check them out one-by-one on their websites. Submission policies are always listed at reputable places.