We study the positivity and regularity of solutions to the fractional porous medium equations× Ω, for m > 1 and s ∈ (0, 1) and with Dirichlet boundary data u = 0 in (0, ∞) × (R N \ Ω), and nonnegative initial condition u(0, ·) = u 0 ≥ 0. Our first result is a quantitative lower bound for solutions which holds for all positive times t > 0. As a consequence, we find a global Harnack principle stating that for any t > 0 solutions are comparable to d s/m , where d is the distance to ∂Ω. This is in sharp contrast with the local case s = 1, where the equation has finite speed of propagation.After this, we study the regularity of solutions. We prove that solutions are classical in the interior (C ∞ in x and C 1,α in t) and establish a sharp C s/m x regularity estimate up to the boundary. Our methods are quite general, and can be applied to wider classes of nonlocal parabolic equations of the form u t + LF (u) = 0 in Ω, both in bounded or unbounded domains.