CRISPR-Cas9 is a genome-editing technology that allows scientists to make targeted changes to DNA. Because many genetic diseases are caused by single mutations, CRISPR has been proposed as a way to prevent inherited disorders at the embryonic stage. However, when gene editing is performed in human embryos, the changes become part of the germline and are passed to future generations. This raises both scientific and ethical concerns. In this presentation, I first explain how CRISPR-Cas9 works at the molecular level, including guide RNA targeting, DNA cleavage, and the cellular repair pathways that follow. I focus especially on how rapid cell division in early embryos can lead to mosaicism and unintended mutations. Second, I examine historical and bioethical implications, including the 2018 case of embryo gene editing that prompted global concern. This case demonstrated how scientific capability can advance faster than ethical consensus and regulatory oversight. Finally, I discuss alternative approaches, such as preimplantation genetic testing, and newer precision editing technologies that aim to reduce DNA damage. Overall, while CRISPR-Cas9 represents a major advancement in genetic medicine, its use in human embryos remains scientifically limited and ethically complex.