Justification of conversions to cellular manufacturing must show that cells are either a more cost effective way to obtain the improvements desired, or that the desired performance improvements cannot be achieved through improvements to the existing system. Although the model-based literature that compares the throughput time performance of functional and cellular layouts has identified conditions that are important for these conversion decisions, the question remains whether the underlying factors in these studies are the same as those relied on by industry when making cell conversion decisions. If not, what other factors should be considered? This study uses information obtained from case studies of four manufacturing plants to address these issues.