Grounded in Self-determination Theory (Deci & Ryan, 2000), this thesis investigated through two studies the reasons for which individuals engage in pro-environmental behaviors. Study 1 with N = 375 Turkish young adults (Mage = 22.35; SD = 2.38), showed that after controlling for connectedness to nature and perceived environmental threat, autonomous reasons but not controlling reasons had unique associations with both private- and public-sphere behaviors and intentions. Building on Study 1, Study 2 employed a more dynamic approach to examine the week-to-week relations of autonomous and controlling reasons to pro-environmental behaviors. With the aid of a sample of young adults (total N = 160; Mage = 23.55; SD = 7.17) who completed two sets of online surveys, a pre-diary part and a series of short questionnaires for six consecutive weeks, it was found, again, that weekly variation in autonomous reasons (but not in controlling reasons) related positively to weekly fluctuation in private- and public-sphere pro-environmental behaviors. Further, perceived environmental threat predicted between-person differences in such behaviors exhibited in both realms. The findings and their implications were discussed within the framework of Self-determination Theory.