The heterogeneity of myelodysplastic syndromes (MDS) has made evaluating patient response to treatment challenging. In 2006, an International Working Group (IWG) proposed a revision to previously published standardized response criteria (IWG 2000) for uniformly evaluating clinical responses in MDS. These IWG 2006 criteria have been used prospectively in many clinical trials in MDS, but proved challenging in several of them, especially for the evaluation of erythroid response. In this report, we provide rationale for modifications (IWG 2018) of these recommendations, mainly for "hematological improvement" criteria used for lower risk MDS, based on recent practical and reported experience in clinical trials. Most suggestions relate to erythroid response assessment, which are refined in an overall more stringent manner. Two major proposed changes are the differentiation between "procedures" and "criteria" for hematologic improvement-erythroid (HI-E) assessment and a new categorization of transfusion burden subgroups.