In many inflammatory diseases, the cellular components in body fluids [cerebrospinal fluid (CSF), serous fluids] are increased, rendering essential diagnostic information. The diagnostic value of the total white blood cell count (WBC) and differential count has been evaluated extensively over the years, and a remarkable amount of knowledge has been gained; yet, there is a great deal of clinical uncertainty whether the diagnosis should be based solely on these variables. In some diseases, such as peritonitis, the total WBC and differential count has high sensitivity; whereas, in differentiating pleural effusions, it lacks the sensitivity required to be clinically useful. Nevertheless, many guidelines consider these tests as cornerstone parameters, and in combination with clinical variables, they can successfully guide clinical decision making in initiating or postponing a treatment course for infection and/or inflammatory diseases while awaiting culture results. Although other methods are available for detecting and differentiating WBCs in body fluids, manual microscopy is still considered the gold standard despite its many limitations. During the last decade, automated analyzers have become a popular method for first line screening. Continued progress in their design has led to major improvements including their speed, improved accuracy and lower variability compared with microscopy. Disadvantages of this method include high imprecision in low ranges (depending on the method) and interfering factors. In a time where automation is at the front line in clinical laboratories, it is essential the results obtained are precise, accurate and reproducible. This review provides an overview of the relevance for cell counting in a variety of diagnostic body fluids, and highlights the current technologies used.