Metals are an essential part of the life of all organisms because they participate as an essential part of diverse components, especially as enzymatic cofactors. In humans, there are metals that are trace elements and therefore are required for the proper functioning of different biological processes, so they must be present in cells and tissues. However, when the organism is overexposed, those same essential metals—in high concentrations that become toxic—cause imbalances or overt pathologies. On the other hand, there are metals that are not essential in humans, so their presence and accumulation in the organism can cause adverse effects. In this review we focus on the essentiality and toxicity of the main trace metals such as iron, zinc, copper, manganese, chromium, cobalt, molybdenum, and nickel, as well as on the toxicity of metals such as vanadium, cadmium, and lead that are not essential for humans. In addition, the report describes the main mechanisms by which metals exert their toxic effects on the body, as well as the primary sources of pollution through which they are released into the environment.