Power BI developers need proficiency in DAX (Data Analysis Expressions) for writing complex calculations and Power Query for data transformation. Knowledge of SQL for querying databases is crucial, as is familiarity with data modeling principles to create efficient data relationships. Power BI developers should also be skilled in creating interactive visualizations and dashboards. Experience with ETL (Extract, Transform, Load) processes and integrating data from various sources, like SQL Server, Azure, and Excel, is important. Familiarity with Power BI Service for sharing reports and setting up data refreshes is also valuable.