Cancer remains a major burden in our modern society, being the second leading cause of death worldwide. Many efforts are being made to improve our knowledge about the etiology of the disease, its early diagnosis and treatment. Innovative technologies capable of early detection, diagnosis and monitoring the progression of cancer, as well as the efficiency of the therapies, are needed. Microfluidic devices can provide the needed conditions to boost the investigation in cancer theranostics due to their advanced capabilities and versatility. During the last few years, microfluidics systems have been employed in cancer research for the development of in vitro tumor models to investigate the mechanistic determinants of the disease, for drug screening and discovery applications, or for the fabrication of point-of-care devices for pre-clinical uses. Herein, we review the contribution of microfluidics in cancer research, ranging from fundamental research to clinical and industrial applications. We also discuss about the challenges that microfluidic devices must overcome to be adopted by the market and the society, as well as the future directions in cancer research.