Ukraine’s military on Monday said that its air defense forces destroyed 76 out of 104 drones launched by Russian troops since Sunday evening, while Russia claimed its air defenses intercepted 21 Ukrainian drones overnight.

Many of the drones aren’t particularly fast or high-tech, but they are cheap enough for the Kremlin to launch more than 700 in one night, in an effort to overwhelm Ukraine’s air defenses. Here’s what we know.