Hammer drills pulse the bit forward as they turn to break up hard but brittle materials like concrete, brick, etc. Technically these are rotary hammer drills since the bit is rotating as well. A true hammer drill would only pulse the bit back and forth to break up the concrete. If you need a true hammer drill you can find a recommendation on our 36 volt page.
But most cordless hammer drills are rotary drills and so that's what I'm reviewing here. Lithium-ion batteries have advanced this category significantly. It takes a lot of battery power as well as muscle to drill through concrete. Older drills had the muscle but would give out way too soon. You'd spend all of your time changing out batteries. But now we have drills that run for a while letting you get some work done.
The best cordless hammer drill out there, IMHO, is the Milwaukee 0726-22 M28 1/2-Inch Hammer Drill . The is a newer model of my previous recommendation with better stats. I haven't been able to use one yet, but I've talked to a professional who's used one extensively and he says it's only gotten better. This 28 volt model has the battery life and power to do some serious work. It's also pricier than most 18v models. If you're going to use this drill for your livelihood you should spring for the M28 system. It will be worth it in saved time. If you're budget conscious and don't do that much hammer drilling there's also an 18 volt model .