Fixation is usually the first step to prepare biological material for microscopy. Any physico-chemical procedure which can preserve the structure or biochemical composition of cells is considered to be a fixative. The quality of fixation will influence every subsequent step. To this aim, preservation of cells with minimal alteration of morphology is an essential part and, of course, includes virtually no loss of tissue molecules. Hence, the purpose of tissue fixation is to preserve the cells in as life-like a state as possible. Furthermore, fixation is necessary to protect the biological specimen from denaturing effects of dehydration and subsequent processings. The choice of a fixative and fixation protocol from a large number of possible schedules depend on the subsequent steps and analyses that are planned. All processes of tissue preparation should be as reproducible as possible.