The depletion of dust grains in the ionized gas is predicted by models and observed both in HII regions and in the diffuse medium. However, the mechanism responsible for this depletion has yet to be clearly identified. After a brief review of the possible depletion scenarios, I will present the results of the analysis of a sample of Galactic evolved HII regions performed by combining IRAC 8 micron, MIPS 24 micron, PACS 70/160 micron and SPIRE 250, 350, 500 micron data. The analysis shows that dust inside HII regions is depleted mostly due to radiation pressure driven drift, as recently proposed by Draine (2011). In addition, we also find that radiation pressure induces a selection of the size of the grains, with the surviving grains being either small Big Grains (a ~ 0.01 micron) or large PAHs (a ~ 0.005 micron). In the last part of my talk, I will discuss implications of studies of HII regions for Anomalous Microwave Emission (AME), a recently discovered emission mechanism commonly attributed to spinning dust.