In machining processes, tool performance is measured by the tool life, which is determined by the tool wear rate. This rate is strongly dependent on the tool wear mechanisms that occur in a specific process. Moreover, determining the wear mechanism is a fundamental task for the development of cutting tools. However, the tool wear mechanism depends on factors such as the workpiece material, the cutting operation, the properties of the tool material, the cutting conditions, and the cooling/lubrication system. This study aims to contribute to the understanding of the mechanisms that cause wear in tools. First, it presents a review of the literature describing the wear mechanisms that are present in metal cutting. Then, an analysis of the tool wear mechanisms during the machining of steels and stainless steels using different cutting tools is presented based on a review of studies performed mostly by research groups at the University of Campinas and Federal University of Uberlandia, Brazil. This analysis was performed using pictures of the worn regions taken in a scanning electronic microscope (SEM) with energydispersive spectroscopy (EDS) device which are fundamental to better understand the causes of tool wear. The major findings of this work were as follows: (i) Attrition is a very important wear mechanism in the machining of ductile materials such as steels and stainless steels; (ii) when a steel alloy with high ductility and work hardening rate is machined, a hard burr occurs in the end of the depth of cut and the burrfurrowing effect on the tool coating stimulates the attrition mechanism; and (iii) other wear mechanisms like abrasion and diffusion also appear in the machining of steels and stainless steels.