  The effect of an electric field on conduction in a disordered system is an
old but largely unsolved problem. Experiments cover an wide variety of systems
- amorphous/doped semiconductors, conducting polymers, organic crystals,
manganites, composites, metallic alloys, double perovskites - ranging from
strongly localized systems to weakly localized ones, from strongly correlated
ones to weakly correlated ones. Theories have singularly failed to predict any
universal trend resulting in separate theories for separate systems. Here we
discuss an one-parameter scaling that has recently been found to give a
systematic account of the field-dependent conductance in two diverse, strongly
localized systems of conducting polymers and manganites. The nonlinearity
exponent, \textit{x} associated with the scaling was found to be nonuniversal
and exhibits structure. For two-dimensional (2D) weakly localized systems, the
nonlinearity exponent \textbf{\textit{x}} is $\geqslant 7$ and is roughly
inversely proportional to the sheet resistance. Existing theories of weak
localization prove to be adequate and a complete scaling function is derived.
In a 2D strongly localized system a temperature-induced scaling-nonscaling
transition (SNST) is revealed. For three-dimensional (3D) strongly localized
systems the exponent lies between -1 and 1, and surprisingly is quantized
(\textit{x} $\approx$ 0.08 \textit{n}). This poses a serious theoretical
challenge. Various results are compared with predictions of the existing
theories.
