As reported in research from abroad, athletes use many dietary supplements that contain substances with unconfirmed performance effect or even substances that have a negative effect on performance. Since there are no data available about the use of dietary supplements among Slovenian athletes, we tried to provide the first insight into the prevalence of dietary supplement use among Slovenian athletes. In the first part of the thesis, the prevalence of supplements was estimated from anonimised data stated on the Doping Control Form of the Slovenian Antidoping Organisation, which also includes information on the dietary supplement use in the last seven days prior to doping control. In the second part of the thesis we used an online questionnaire to obtain data about the average number of dietary supplements consumed by Slovenian athletes, type of supplements used, and other specific data about the dietary supplement use. In the third part of the thesis we examined whether the use of dietary supplements is grounded or not with the method of a 3-day nutritional diary. The results demonstrate that as much as 69 % of the athletes involved in the first part of the study (N = 389/561) used dietary supplements in the 7 days prior to completion of their Doping Control Form. The most commonly used supplements by the Slovenian athletes were proteins (38 %; N=148/389), minerals (28 %; N=107/389) and vitamins (26 %; N=103/389). After grouping of protein and vitamin supplements to wider categories, proteins are still most commonly used (56%), followed by vitamin supplements (43%) and minerals as third most common supplement (28%). Most athletes included in the second part of the thesis (48 %; N = 39/81) decided to use dietary supplements based on their own knowledge, while the second most common reason was the advice of their coach (32 %; N=26/81). In the last part of the thesis we observed that some of the dietary supplements, used by the three athletes who completed their 3-day nutritional diary precisely enough, could be beneficial (provided they are of high enough quality and irreproachable) in counteracting the deficiency of a particular nutrient, present due to unbalanced diet; yet, it was obvious that sportsmen used some of the dietary supplements groundlessly. The results of the thesis demonstrate that education of sportsmen and their coaches about different aspects of dietary supplement use should be improved.