Explains Generative AI, focusing on Large Language Models (LLMs), distinguishing them from predictive AI and detailing their complex ecosystem of interacting components. It then comprehensively discusses the unique challenges of evaluating LLMs, outlining various evaluation types and specific metrics, presenting best practices for continuous assessment, and explaining how Vertex AI facilitates this process through both computation-based and model-based methods like Auto Side-by-Side (AutoSxS)