Dust traps are the most promising mechanisms to explain the observed substructures in protoplanetary discs. In this talk, I will present recent ALMA observations of the discs around LkCa 15 at 3 mm and PDS 70 at 0.4 mm, which, together with multi-wavelength archival data, help us understand how dust grains are effectively trapped in the rings of LkCa 15, whereas in PDS 70 only the large grains (mm grains) are trapped, while the small grains (sub-mm grains) escape the trap both radially inward and outward, filtering through the cavity and feeding the inner disc.
