Many individuals believe that life gets better and better over time. To examine the sources and significance of such beliefs, we examined lay theories for life satisfaction (LS) in relation to individuals’ beliefs concerning how their LS was unfolding over time. Two studies were conducted with online participants: one correlational (Study 1; N = 320, M age = 30.39, 55% male), the other experimental (Study 2; N = 321, M age = 30.46, 53% male). In both studies more incremental (vs. entity) lay theories were associated with more steeply inclining subjective trajectories for LS. Furthermore, both sets of beliefs had unique effects on individuals’ goal-striving toward a brighter future life, as well as psychological adjustment (self-efficacy, hope, optimism, positive affect, negative affect). Thus, lay theories and subjective trajectories for LS share a common assumption concerning change in life satisfaction over time. And each set of beliefs plays a unique role in positive functioning.