What is the difference between a burr grinder and a blade grinder for espresso?

A burr grinder crushes coffee between two abrasive surfaces to produce consistently sized particles — essential for espresso. A blade grinder chops randomly, producing a mix of fine dust and large chunks that extract unevenly and make good espresso nearly impossible. Never use a blade grinder for espresso. For pour over and drip, blade grinders are tolerable; for espresso, a burr grinder is a non-negotiable requirement.