A Magnetically doped amorphous Si provides a striking comparison to the crystalline magnetic semiconductors more commonly studied. Amorphous (a-) GdxSi1-x alloys are extraordinarily sensitive to magnetic field and temperature, with a negative magnetoresistance of many orders of magnitude (>10^5 at 1K, 1% at 90K). Tunneling and IR spectroscopy measurements also are strongly dependent on magnetic field, showing evidence of many body correlation effects (e.g. spectral weight is not conserved with field or temperature out to the Si band edge). Magnetic interactions between Gd ions have strength >125K, comparable to GaMnAs or Gd metal, but are mixed ferro- and antiferromagnetic leading to spin glass freezing. The magnetic properties are surprisingly complex, compared to the usually simple properties of Gd in both metallic and insulating alloys, reflecting the influence of the semiconducting matrix. As an example, the susceptibility follows nearly a simple Curie Law above the spin glass freezing, but the magnitude is strongly and non-monotonically dependent on composition, with a peak at the metal-insulator transition (at 14 at.% Gd). There is a fairly clear characteristic temperature below which the Gd magnetic moments affect the electrical transport; this temperature is high (near 100K) and shows a surprising and only partially understood dependence on semiconductor matrix (Si vs Ge) and rare earth concentration (including ternary Gd-Y-Si alloys). By contrast, Mn-doped a-Si is non-magnetic and show no significant magneto-transport effects. While Mn-doped a-Ge shows simple magnetic properties, for reasons that can be qualitatively understood.