In order to function properly, every living organism must have favourable conditions for its operation, i.e. it must be systematically supplied with the necessary nutrients. On the example of selenium (Se), it can be seen how important is the right balance between providing the right amount of it, without exceeding the level above which it is toxic. The amount of Se in soil is closely correlated with its content in the parent rock; therefore, it differs depending on the soil type and may change even in a small area. Considerable dispersion of the element in the soils of Poland is related to their formation, mainly from dump materials of various glaciations. The problem of maintaining the balance between deficiency and excess of Se in the diet of humans and animals is related primarily to the uneven distribution of this element in nature. In this paper, on the basis of scientific literature, the current issues related to the deficiency and excess of Se in the soil and its possible sources are presented. The relationship between the content in the soil and the supply of Se in plants, animals and humans was also shown. The aim of this study was to summarise the state of knowledge on the complexity of Se occurrence in the environment and its importance in the soil-plant-animals-human system.