Why does my espresso have a sour taste?

A freshly pulled espresso that tastes sour is usually a result of under-extraction. It could be that:

It could be that:

• Your espresso shot is pulling too quickly due to 'channeling'. That means the water is finding a weak point in the coffee puck and flowing too quickly for proper extraction to take place. Make sure you're tamping evenly on a level surface.
• Water is passing through too quickly due to the grind size being too coarse. So, try a finer grind. This will increase the surface area of the coffee, making it easier for the water to extract all the flavours.