Seed germination is a drastic developmental transition taking the plant from a highly protected, desiccated and quiescent form of life (dry seed) into a more fragile, vegetative seedling. Seed germination is tightly controlled by the environment, which determines the relative levels of two phytohormones: GA (gibberellins) and ABA (abscisic acid). Consequently, GA and ABA are key regulators of Arabidopsis seed germination. GA stimulates germination and its synthesis upon seed imbibition is necessary for germination to take place. On the other hand, environmental conditions unfavorable for seed germination trigger ABA synthesis, which in turn arrests germination. Until now, metabolic and signaling pathways of these two hormones were studied separately. The purpose of my thesis was to re-examine the role of GA and ABA on the control of seed germination in order to understand how relative levels of these two hormones are coordinated within the seed.