  Liquid 3He at low temperatures is an ideal substance to study because of its
natural purity. The superfluid state, which appears at temperatures below 3 mK,
has many unusual and exciting properties. These have extensively been studied
experimentally, and the results are in many cases well understood theoretically
using quasiclassical approximation. In this work we study the case that liquid
3He is inside of aerogel. Aerogel is a very porous substance, where 98% of the
volume can be empty. The purpose is to understand how the properties of the
superfluid are modified when the quasiparticles are scattered from an impurity,
such as aerogel. We study extensively the "homogeneous scattering model". From
it we derive the Ginzburg-Landau theory of impure superfluid 3He. We give
expressions for measurable quantities and compare them with experiments. We
consider random anisotropy and its effect on the NMR properties. More
sophisticated scattering models are briefly discussed.
