Why does my espresso taste sour?

Sour espresso is almost always under-extracted. Common causes: grind too coarse (water flows through too fast), water temperature too low, dose too light, or shot pulled too short. Fix: grind finer first (most common fix), ensure machine is fully preheated, check that you're using the correct dose for your basket size. A shot that runs in under 20 seconds with a sour taste is a textbook under-extraction — finer grind is the first adjustment to make.