Fixation is one of the most critical steps in immunostaining. The object of fixation is to achieve good morphological preservation, while at the same time preserving antigenicity. Tissue blocks, sections, cell cultures or smears are usually immersed in a fixative solution, while in other situations, whole body perfusion of experimental animals is preferable. Fixation can be accomplished by either chemical or physical methods. The chemical methods include cross-linking agents such as formaldehyde, glutaraldehyde and succinimide esters as well as solvents such as acetone and methanol, which precipitate proteins. Of the physical methods, freezing tissue and air drying are most widely used. This chapter deals with the chemical fixation methods most commonly used for light microscopy.