According to both USA cGMP1 and EU GMP2 cleanroom classification should be carried out according to ISO14644-1. The 19994 version of this ISO standard has been re-written by a group of international subject matter experts and ISO 14644-1:20156 was published on December 15, 2015. ISO 14644-1:2015 contains substantial revisions impacting both the way the cleanrooms are classified and the performance requirements of the air particle counting instruments used to carry out the classification. Cleanroom users claiming compliance to cGMP and/or GMP will have to consider these new rules when next re-qualifying their cleanrooms, potentially purchasing new air particle counters if their existing counters do not comply with the new performance rules and facing the dilemma that the new re-qualification may indicate that their cleanroom is not operating at the grade they had achieved using the old classification process. This paper explains the new rules.