The separation of the macromolecules takes place in their interior. They are filled with a porous crosslinked polymer, and the macromolecules to be separated interact with the polymer pores depending on their size in solution. It is highly recommended to use at least a set of three columns in order to obtain good results. Columns are available at different pore sizes. Columns with very small pores sizes are used for polymers of low MW, while columns packed with material containing large pore sizes are used for polymers with high MW. High efficiency columns give maximum separating capability and rapid analyses. Every column must provide reproducible information over extended periods for both analytical and fraction-collection purposes. There are columns for different types of applications. For example, there are columns that can stand high temperatures, which are used to fractionate polymers that are soluble only at high temperatures, such as polyethylene or polypropylene. There are also columns suitable to work with aqueous solvents, those columns are packed with a material known as ultrahydrogel, which is basically a crosslinked hydroxylated polymethacrylate; these columns are used with polymers that are soluble in water, such as poly(acrylic acid), poly(vinyl alcohol), poly(ethylene glycol), etc.