Silica aerogels are sol-gel derived nanostructured materials with high porosities up to 99.8%. The nanostructures of silica aerogels are investigated by BET, SEM, SAXS techniques. A typical fractal structure is found especially for base-catalyzed aerogels. That is the aerogels are made of particles of size a, these particles form the branching and interconnecting networks up to an average cluster size ξ, At length scales above ξ, the material is an homogeneous assembly of such clusters; between a and ξ, the structure is fractal. Meanwhile, the effects of catalysis and aging on the structures are investigated and discussed. The reasons why SCD technique should be used are also deeply discussed.