In ovo delivery of nutrients and bioactive compounds during embryonic development is a promising technique to support broiler health, resilience, and performance in later life. However, variable outcomes across studies highlight the need for hypothesis-driven research and methodological consistency. This chapter outlines key factors influencing success of this technique, including dose, volume, timing, injection compartment, compound properties, and external conditions. A case study on essential oils highlights how methodological factors - such as delivery compartment, emulsifier choice, and compound dynamics - can be systematically addressed to optimise outcomes. Despite potential, in ovo delivery of nutrients and bioactive compounds often yields context-dependent results and may primarily benefit chickens under suboptimal conditions. Integration with in ovo delivery of vaccines could improve practicality, but further research is needed to ensure scalability, efficacy, and long-term impact. Future research should prioritise defined hypotheses, mechanistic endpoints, and realistic production scenarios to realise the full potential of this approach.