Anthropogenic climate change stressors including ocean warming, acidification, and deoxygenation have decimated populations of cnidarians (e.g., corals and sea anemones) on a global scale over recent decades. Conserving and restoring cnidarian populations requires a thorough understanding of these species’ early life processes (i.e., reproduction and development), yet we lack foundational knowledge in this area, making it an urgent topic for investigation. In particular, understanding how climate change stressors influence cnidarian gametes and early life stages (e.g., larvae and juveniles) is critical to building our predictive and conservation capacities for these species. This dissertation broadly explores mechanisms underpinning cnidarian reproduction and development, as well as the influence of climate change stressors on these processes. First, we found that an evolutionarily conserved, pH-dependent signaling pathway involving the enzyme soluble adenylyl cyclase (sAC) controls sperm motility in the temperate coral Astrangia poculata. Notably, the fact that this pathway relies on the alkalinization of the sperm cytosol suggests the potential for reduced sperm motility under ocean acidification for A. poculata and other taxa. Next, we found that exposure to intermediate (but not low or high) acute heat stress improved short-term climate resilience (e.g., heat tolerance) in early life stages of the sea anemone Nematostella vectensis, demonstrating hormetic conditioning. We also found that exposure to seawater hypoxia disturbed aerobic metabolism leading to disrupted growth, development (e.g., decreased settlement), and physiology (e.g., decreased larval swimming) in early life stages of N. vectensis and two reef-building corals, Galaxea fascicularis and Porites astreoides. Finally, we found that exposure of adult N. vectensis to seawater acidification or hypoxia induced changes in gamete production and physiology (e.g., decreased egg production), which were correlated with modified outcomes (e.g., accelerated development) in unexposed offspring, reflecting intergenerational plasticity. Overall, this body of work contributes foundational insights to our understanding of the sensitivity and resilience of cnidarian early life processes to climate change. These findings broadly affirm the need for a rapid reduction in anthropogenic greenhouse gas emissions for the long-term survival of these invaluable taxa.