  The Hubbard model on a two-leg ladder structure has been studied by a
combination of series expansions at T=0 and the density-matrix renormalization
group. We report results for the ground state energy $E_0$ and spin-gap
$\Delta_s$ at half-filling, as well as dispersion curves for one and two-hole
excitations. For small $U$ both $E_0$ and $\Delta_s$ show a dramatic drop near
$t/t_{\perp}\sim 0.5$, which becomes more gradual for larger $U$. This
represents a crossover from a "band insulator" phase to a strongly correlated
spin liquid. The lowest-lying two-hole state rapidly becomes strongly bound as
$t/t_{\perp}$ increases, indicating the possibility that phase separation may
occur. The various features are collected in a "phase diagram" for the model.
