We investigate the viability of the magnetorotational instability ( MRI ) in X-ray ionized viscous accretion disks around both solar-type stars and very low mass stars . In particular , we determine the disk regions where the MRI can be shut off either by Ohmic resistivity ( the so-called Dead and Undead Zones ) or by ampipolar diffusion ( a region we term the Zombie Zone ) . We consider 2 stellar masses : M _ { \ast } = 0.7 M _ { \odot } and 0.1 M _ { \odot } . In each case , we assume that : the disk surface density profile is that of a scaled Minimum Mass Solar Nebula , with M _ { disk } / M _ { \ast } = 0.01 as suggested by current data ; disk ionisation is driven primarily by stellar X-rays , complemented by cosmic rays and radionuclides ; and the stellar X-ray luminosity scales with bolometric luminosity as L _ { X } / L _ { \ast } \approx 10 ^ { -3.5 } , as observed . Ionization rates are calculated with the moccasin Monte Carlo X-ray transport code , and ionisation balance determined using a simplified chemical network , including well-mixed 0.1 \mu m grains at various levels of depletion . We find that ( 1 ) ambipolar diffusion is the primary factor controlling MRI activity in disks around both solar-type and very low mass classical T Tauri stars . Assuming that the MRI yields the maximum possible field strength at each radius , we further find that : ( 2 ) the MRI-active layer constitutes only \sim 5–10 % of the total disk mass ; ( 3 ) the accretion rate ( \dot { M } ) varies radially in both magnitude and sign ( inward or outward ) , implying time-variable accretion as well as the creation of disk gaps and overdensities , with consequences for planet formation and migration ; ( 4 ) achieving the empirical accretion rates in solar-type and very low mass stars requires a depletion of well-mixed small grains ( via grain growth and/or settling ) by a factor of 10–1000 relative to the standard dust-to-gas mass ratio of 10 ^ { -2 } ; and ( 5 ) the current non-detection of polarized emission from field-aligned grains in the outer disk regions is consistent with active MRI at those radii .