Synucleins are a family of small, soluble proteins mainly expressed in neural tissue and in certain tumors. Since their discovery, tens of thousands of scientific reports have been published about this family of proteins as they are associated to severe human diseases. Although the physiological function of these proteins still elusive, their relation with neurodegeneration and cancer was clearly described over the years. In this review, we summarize data connecting synucleins and cancer, going from the structural description of these molecules, to their involvement in tumor-related processes, and discuss the putative use of this proteins as cancer molecular biomarkers.