Project management institute (PMI) requires the certified professionals , to renew their credentials every 3 years. You need to collect PDUs in different categories. This applies to all professional certifications, namely PMP, PMI-RMP, PMI-SP, PMI-ACP , PgMP and so on. I have tried to collate all the possible sources to get Free PDUs, if you know of some more sources, feel free to share here.