In this issue of Best Practice & Research Clinical Haematology, the focus is on mesenchymal stromal cells (MSC), beginning with a remembrance of Dr. Arnold Caplan, a distinguished scientist who discovered, described, and advanced the science of these important cells. The following articles will impress upon the reader how the field of MSC has grown and developed since being discovered by Dr. Caplan 40 years ago. Drs. Olson, Bukulmez, and Bonfield, experts in hematology, rheumatology, and