Great Expectations is a Python library for data quality validation. Data engineers define 'expectations' (rules about data—nulls, ranges, uniqueness, regex patterns) and run them on data pipelines. When expectations fail, the system alerts engineers before bad data reaches analytics or ML models. Advanced practitioners build organization-wide data quality infrastructure: profiling datasets to auto-generate expectations, tracking expectation pass/fail rates, and integrating with orchestration tools (Airflow, Prefect) and notification systems (Slack, PagerDuty).