The recent approval of 3 drugs for the treatment of myelodysplastic syndromes (MDSs) has resulted in a revolution in therapeutic options that was absent a decade ago. At the same time, the changing MDS environment is raising new challenges in clinical trial design and defining new indications for MDS drugs. Many current trials still rely on IPSS-based enrollment criteria, despite the well-recognized limitations of the IPSS. Clinical trialists designing studies struggle with several important trial design challenges, including which patients constitute the "previously treated" and "relapsed/refractory" MDS populations, and how specifically to define disease "progression." This article considers some of these issues as they relate to study design, including how to identify certain MDS populations and define disease progression.