This article explains how to select the right flange for oil & gas and petrochemical applications based on pressure, temperature, media, standards, materials, and facing type. It compares common flange types such as weld neck, slip-on, blind, socket weld, threaded, and lap joint, and highlights why weld neck flanges are often preferred for critical high-pressure service. It also includes a simple selection process, practical application examples, and common mistakes to avoid.