Planet formation is one of the hottest topics in astronomy especially owing to the recent developments of ALMA observations of protoplanetary disks. Dust grains, which are the seeds of planets, coagulate to form larger and larger bodies, and finally form into thousand-kilometer-sized planets. One of the key observational constraints are on the dust mass and size, which directly constrain planet formation theories. In this talk, I will first review the recent observations of protoplanetary disks especially focusing on the observational constraints on dust grains. Then, I will discuss polarimetric observations of protoplanetary disks with ALMA. One of the dominant mechanisms is now believed to be the self-scattering, which allows us to constrain the dust sizes. However, the new measurements make the tension between the classical understandings of millimeter-sized dust grains and the sub-mm size dust grains with the new polarimetric constraints. I will discuss possible solutions and their impact on planet formation.