We show that the down-up algebras of G. Benkart (1998, in "Recent Progress in Algebra," Contemporary Mathematics Vol. 224, Am. Math. Soc., Providence) and G. Benkart and T. Roby (1998, J. Algebra 209, 305-344) lie in a certain class of iterated skew polynomial rings, called ambiskew polynomial rings, in two indeterminates x and y over a commutative ring B. In such rings, commutation of the indeterminates with elements of B involve the same endomorphism σ of B, but from different sides, that is, yb = σ b y and bx = xσ b , and, for some scalar p, yx − pxy ∈ B. In previous studies of ambiskew polynomial rings, σ was required to be an automorphism but, in order to cover all down-up algebras, this requirement must be dropped. The Noetherian down-up algebras are those where σ is an automorphism and, in this case, we apply existing results on ambiskew polynomial rings to determine the finite-dimensional simple modules and the prime ideals. We adapt the methods underlying these results so as to apply to the non-Noetherian down-up algebras for which they reveal a surprisingly rich structure.