Tissue processing is concerned with the diffusion of various substances into and out of stabilized porous tissues. The diffusion process result from the thermodynamic tendency of processing reagents to equalize concentrations inside and outside blocks of tissue, thus generally conforming to “Fick’s law” the rate of solution diffusion through tissues is proportional to the concentration gradient (the difference between the concentrations of the fluids inside and outside the tissue) as a multiple of temperature dependent constants for specific substances. This chapter covers the selection and labeling of tissues, completion of fixation before processing, post-fixation procedures, dehydration, clearing, impregnation and infiltration, automatic tissue processing, manual tissue processing, paraffin wax embedding, storage of paraffin blocks, celloidin and low viscosity nitrocellulose embedding, gelatin embedding, plastic embedding, resin embedding, and double embedding. In this chapter, questions for exercise and objective type questions are also provided.