Three schools of thought. Tumble before you resize/deprime, as the tumbling media has a tendency to get stuck in the flash hole. This means if your primer pockets are especially dirty, you will need to brush out the pockets to ensure proper seating and ignition. Tumble after you resize/deprime to get the primer pockets clean as well. As mentioned above, you may get a chunk of cleaning media stuck in the flash hole. Each piece of brass must be inspected to ensure that the hole is clear. Proponents of stainless steel tumbling media use this as one reason to buy the pricey stuff. Ultrasonic cleaners (what I'm using now) clean the whole brass, inside and out, WAY cleaner than I've seen standard tumbling media get it. Resize/deprime prior to using these. An issue is that most US cleaners are too small for the amount of brass I like to tumble. After looking at all the jewelers and firearms marketed US cleaners, I got the Hornady Magnum version. Then one day on Amazon, I found HUGE US cleaners, that would hold probably thousands of rounds of handgun brass and nearly any disassembled firearm we own. The price wasn't too much higher, considering the increase in capacity and capability. Hope this helps! Jon