The seemingly simplest property of any material and aerogels especially is the density, defined as the ratio of mass and volume. For any regularly shaped body such as a cube, sphere or cylinder, the volume is readily determined and the mass obtained by simply weighing the body. For a porous material, especially if the shape is not regular, the density is not that easy to determine. For aerogels, two different values are usually determined: the so-called envelope densityandthe skeletal density. The envelope density is defined as the massdivided by the total volume enclosing the porous structure. The skeletal density instead is the density of the solid backbone of the aerogel, i.e., the sum of the volume of all nanoparticles making up the aerogel. The chapter discusses techniques to measure both densities and all aspects of these techniques and closes with a section discussing how to estimate the final aerogel density from the known composition of the monomeric precursor solution or in the case of biopolymers that of a polymeric solution.