This paper gives asymptotic formulas for the subgroup growth and maximal subgroup growth of all Baumslag-Solitar groups.

I completed much of the work in this paper while I was a graduate student at Binghamton University. See [5], my dissertation. Also, I would like to thank the referee for pointing out that Lemma 2 of [3] shortens my proof of Theorem 4.7 and for showing me the present proof of Theorem 5.3. (Longer proofs of these theorems, which I found, are in the arXiv version of this paper.)