This chapter provides descriptions and examples of methods for the statistical evaluation of data from virus plaque counting assays. The primary emphasis is on techniques for evaluating the precision of estimates obtained from such assays, in particular, titer estimates of total plaque forming units (PFU), PFU titer by type of virus, and relative frequencies of virus types. Also presented are techniques which should prove useful in the area of quality assurance, namely, methods for evaluating dispersion of the plaques, examining differences between subsets of plaque counts, sceening the data for outliers and a discussion of sample design, based on the concepts of precision presented in the previous sections.