Study aim: Effects of caffeine, L-theanine, and their combination on the physical performance of adolescent soccer players compared to a placebo. Design: A randomized, double-blind, placebo-controlled, four-condition crossover clinical trial will be conducted in 16 trained male adolescent soccer players. Each participant will receive all four conditions: caffeine, L-theanine, caffeine plus L-theanine, and placebo. Participants will be randomly allocated to one of four treatment sequences using a simple lottery procedure with identical cards, with four participants assigned to each sequence. Settings and conduct: This study will be conducted at Zob Ahan Football Club in Isfahan, Iran. Eligible participants will complete four experimental sessions separated by a 7-day washout period and will receive one of the following conditions at each session. Following administration of the intervention, performance tests including sprint, jump, change-of-direction, and repeated-sprint tests will be performed. Participants, performance assessors, and the statistical analyst will be blinded to treatment allocation, with experimental conditions identified using treatment codes. Participants/Inclusion and exclusion criteria: Male participants aged 16 to 18 with a body mass index (BMI) of 18.5 to 24.5, at least two years of regular soccer training experience, physician-confirmed general health, and no known sensitivity to caffeine or L-theanine. Intervention groups: Participants will undergo four experimental conditions (in a crossover design), caffeine (3 mg/kg body mass), L-theanine (3 mg/kg body mass), caffeine plus L-theanine (3 mg/kg body mass each), and placebo. All interventions will be administered as capsules 60 minutes before testing. Main outcome variables: 20-meter sprint time; standing long jump distance; change-of-direction test time; repeated sprint time.