AI systems, due to their stochastic nature, require a unique approach to testing and validation, distinct from traditional deterministic software. Langfuse provides a framework for running and interpreting experiments, allowing developers to evaluate AI applications systematically. This involves defining tasks, utilizing datasets, and employing evaluators to score output quality, while managing factors like cost and latency. The guide emphasizes a structured approach akin to a CI pipeline for model quality, where experiments are executed and results are interpreted through a top-down funnel of macro metrics, baseline comparisons, and root cause analysis. Human annotation plays a critical role in refining automated evaluations, turning regression signals into structured datasets for further iterations. This systematic evaluation process is essential for developing reliable and high-quality AI applications.