This chapter describes the methods found useful for the study of cell division in early cleavage stage sea urchin zygotes. Successful and reliable fertilization of sea urchin eggs is simple if a few precautions are kept in mind. First, use only clean glassware that has not been exposed to fixatives or metal ions. If the glassware is washed in a central facility, be certain that there is no residual detergent present. There have been good results with the use of “conditioned” glassware that is dedicated to use with live cells. At the end of the day rinse the glassware with distilled water and store it filled with distilled water. The glassware should be thoroughly washed if it is exposed to drugs. To conduct routine fertilizations prepare a sperm solution adding just enough sperm to the test tube to make the water barely opalescent. If the sperms are too concentrated, there is the risk of polyspermy. A dilute solution of eggs is prepared and a small amount of sperm solution is added. There is no precise ratio that needs to be attained, but the water around the eggs should appear clear to the eye.