The white blood cell differential count represents a major portion of the workload in the hematology laboratory. The traditional procedure whereby a technologist examines a Romanowsky-stained blood film and classifies 100 cells is both labor intensive and imprecise. To significantly improve the precision, the technologist would have to count and classify at least 1000 cells, which is obviously impractical in terms of time and efficiency. Until the availability of the automated hematology instruments, it was not possible to obtain a differential count that could be regarded as statistically significant. Automation is therefore necessary and desirable for both economic and clinical reasons. Automation can also provide a number of instrument-derived parameters tha have attained diagnostic significance in their own right. Analytic and technologic advances have occurred with every generation of hematology instruments. As the means of analysis becomes more sophisticated, additional categories of cells may be expected to be classified, and current categories may be more accurately and precisely defined. It is hoped this will lead to even greater clinical utility, as well as provide quality assurance and assessment methods that need to be developed similar to other procedures in the clinical laboratory.