A tamper is a handheld tool used to compress (tamp) ground coffee in an espresso portafilter basket before brewing. It looks like a small pestle, with a flat metal base attached to a handle.
Why tamping matters
Espresso depends on water passing evenly through a uniform bed of coffee under high pressure. Loose grounds let water rush through the path of least resistance, creating channels and an uneven extraction. Properly tamped grounds form a level, dense puck that resists water uniformly across its surface.
The result of good tamping: even extraction, balanced shot, proper crema. The result of bad tamping: gushers, channels, and shots that taste sour and bitter at the same time.
What makes a good tamper
The base must match your basket diameter. Most modern machines use 58mm baskets and 58mm tampers. Some smaller machines need 51mm or 54mm. Using the wrong size leaves a gap around the puck where water can bypass.
The base should be flat (or slightly convex, depending on preference). Weight matters less than people claim, but a tamper with some heft (around 200-400g) is easier to control than something too light.
How to tamp properly
Distribute the grounds first. Tap the side of the portafilter or use a distribution tool to create a flat, even bed. Then tamp straight down with consistent pressure. Pressure should be enough to firmly compress the puck (around 15-20 lbs of force, less than people think) but does not need to be Herculean.
The most important thing is consistency from shot to shot, not absolute pressure. Vary your pressure and your shots will vary unpredictably. Tamp the same way every time and you remove a variable.
Calibrated tampers and other gadgets
Calibrated tampers click at a fixed pressure, removing pressure variation. They are useful for beginners learning to be consistent, less useful once you have built muscle memory. Spring-loaded versions are similar.
Self-leveling tampers (also called palm tampers) auto-correct for tilt. These are genuinely useful because tilted tamps are the single most common cause of channeling.