An embodiment device package includes a fan-out redistribution layer (RDL), a device over and bonded to the fan-out RDL, and a molding compound over the fan-out RDL and extending along sidewalls of the device. The device includes a first functional tier having a first metallization layer and a second functional tier having a second metallization layer. The second functional tier is bonded to the first functional tier. The device further includes an interconnect structure electrically connecting the first metallization layer to the second metallization layer. The interconnect structure includes an inter-tier via (ITV) at least partially disposed in both the first functional tier and the second functional tier, and the ITV contacts the first metallization layer.