Crystal Mackall, MD, Stanford University, Stanford, CA, discusses the challenges posed by solid tumours that liquid tumors do not present. Prof. Mackall suggests that more progress has been made in researching treatments for liquid tumors than solid tumours in recent decades, due to challenges of solid tumors such as heterogeneity, the need for sophisticated targets, and the tumor microenvironment. This interview took place during the 36th Society for Immunotherapy of Cancer (SITC) Annual Meeting in Washington, D.C.