Abstract. We undertake a comprehensive study of the nonlinear Schrödinger equationwhere u(t, x) is a complex-valued function in spacetime Rt × R n x , λ 1 and λ 2 are nonzero real constants, and 0 < p 1 < p 2 ≤ 4 n−2. We address questions related to local and global well-posedness, finite time blowup, and asymptotic behaviour. Scattering is considered both in the energy space H 1 (R n ) and in the pseudoconformal space Σ := {f ∈ H 1 (R n ); xf ∈ L 2 (R n )}. Of particular interest is the case when both nonlinearities are defocusing and correspond to the L 2 x -critical, respectivelyḢ 1 x -critical NLS, that is, λ 1 , λ 2 > 0 and. The results at the endpoint p 1 = 4 n are conditional on a conjectured global existence and spacetime estimate for the L 2x -critical nonlinear Schrödinger equation.As an off-shoot of our analysis, we also obtain a new, simpler proof of scattering in H 1 x for solutions to the nonlinear Schrödinger equation iut + ∆u = |u| p u,