Experimental data on localization in two-dimensional GaAs/AlGaAs heter
ostructures are compared with the scaling theory of localization. All
states are found to be localized at T = 0 and B = 0 in contrast to rec
ent reports of a metal-to-insulator transition in a Si MOSFET sample.
Qualitative agreement with the predicted scaling of the localization l
ength with the elastic scattering length is found at B = 0.