In Study 1, 103 children ages 4 through 10 answered questions about their concept of and belief in luck, and completed a story task assessing their use of luck as an explanation for events. The interview captured a curvilinear trajectory of children's belief in luck from tentative belief at age 4 to full belief at age 6, weakening belief at age 8, and significant skepticism by age 10. The youngest children appeared to think of luck simply as a positive outcome; with age, children increasingly considered the unexpected nature of lucky outcomes and many came to view luck as synonymous with chance. On the story task, younger children attributed a stronger role to luck in explaining events than did older children. Studies 2 and 3 explored 2 potential sources of children's concepts. Study 2 explored adult use of the words luck and lucky, and found that most of this input consisted in using lucky to refer to positive outcomes, although the nature of use changed with the ages of the children. In Study 3, we examined children's storybooks about luck and found them to be rich potential sources of children's concepts.