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Classifying is a way of organizing information, which can be defined as an activity where different objects, concepts or beings are assigned to categories according to a given criterion. The criteria for classification of living organisms have changed over the course of time. In this paper, based on a historical survey, the different ways that have existed for classifying living organisms are presented. Some current discussions regarding how organisms are grouped today are also presented, as well as the premises upon which such classifications are made, such as the notion of common ancestry, hierarchy and divergence, the notions of natural category and natural classifications within the biological sciences.

A new technique for cell counting, which allows to determine the concentration of leukocytes in the internal gingival sulcus, is presented. A method of extraction of fluid from the sulcus, with micropippetes, is used. Concentration of leukocytes is determined by the application of the technique of New Bauer chamber. This method has been already applied, successfully, in two investigative works.