Leukocytosis is one of the most frequent abnormalities in clinical practice. There are many etiologies and its correct interpretation might be a very important tool for the diagnosis of several conditions, either pathological or not. In the article to be discussed below, the authors carried out a review on the subject, addressing aspects related to the development of WBC and their roles, evaluating and classifying leukocytosis, malignant and benign diseases associated to Leukocytosis and how the laboratory might assist in recognizing abnormalities. Microscopy images and figures are not part of the original article and were added by the commentator (all from her personal files).