Using expectancy-value theory, we explored whether parents' perceived expectancies, value, and costs relate to parent involvement in science and math activities. We also explored whether informal learning varied based on child gender and parent's report of having a science, technology, engineering, and math (STEM)-related career. Specifically, we examined the mediating role of parents' STEM expectancies, value, and cost as well as whether parents held a STEM-related career on the outcome of parental involvement. Our sample consists of 208 parents of 3- to 5-year-olds from mostly middle class families of diverse races/ethnicities. Descriptively, 56% of these parents reported reading everyday with their child, but just 35% reported "any" daily STEM activities. Controlling for sociodemographic factors, results revealed that only a parent's rating of STEM value, not expectancies or cost, was directly related to parental involvement in science and math. But maternal report of a STEM-related career was indirectly related to parental involvement in STEM through parents' higher self-efficacy for facilitating informal STEM learning. No significant relations were found for child gender. We discuss implications for supporting parents' involvement in early STEM given these findings that parents who feel empowered to do science and math engage their preschooler in informal STEM learning more often. [This paper was published in "Journal of Applied Developmental Psychology."]