K+ Channels form the largest family among ion channels. Besides regulating many physiological functions, K+ channels, being aberrantly expressed in different types of tumors, affect several hallmarks of cancer. In cancer cells, K+ channel activity regulates cell proliferation, resistance to apoptotic cell death, tumor angiogenesis, invasiveness and metastatic spread. Moreover, being expressed in cells of the tumor microenvironment, K+ channels can also modulate the immune/inflammatory response which contributes to drive cancer establishment and progression. After almost 30 years of studies, some K+ channels are emerging as novel cancer biomarkers, to be employed to stratify patients for either prognostic or predictive purposes. Moreover, it is not remote the time in which it will be possible to target specific K+ channels in cancer for therapeutic purposes. One hindrance for applying a K+ channel-based therapy to cancer is the fact that K+ channel blockers can cause side effects, which often overlap with, but can mask, benefits. We here show some strategies to overcome harmful side effects caused by blocking K+ channels. Once taken into account these strategies, K+ channels may represent suitable and easily accessible cancer biomarkers and targets for therapy. The relevant patents related to K+ channels and cancer are discussed.