Cardiac glycosides (CGs) are approved for the treatment of cardiovascular alterations and their known cellular target is the alpha subunit of the sodium (Na+)/potassium (K+)-ATPase (NKA). Pharmacologically, they represent a well-known generation of drugs for treating cardiovascular problems, thus allowing the investigation of potential dose-dependent side effects. Interestingly, since the end of the 1960s, epidemiological studies have indicated that anti-cancer effects were associated with the regular use of these compounds. Since then, a large body of evidence has been accumulated on the in vitro and in vivo effects of CGs in various experimental models, thus confirming their selective action on cancer cell proliferation and viability. CGs have the potential for targeted therapeutic applications. Many of the anti-cancer activities of these compounds have been linked to the inhibition of their primary target, the NKA. A number of studies have shown a correlation between the overexpression of specific alpha subunits in cancerous versus non-cancerous cells and cancer cell responsiveness. Other findings have provided evidence of the on-target nature of the ascribed anti-cancer effects. More recently, studies have indicated additional intracellular targets for these agents, whose modulation might be, at least in some instances, unrelated to NKA targeting. These include endosomal trafficking of both NKA and Src kinase, downregulation of pro-survival Mcl-1 and Bcl-xL pro-survival proteins, and immunogenic cell death induction, among others. This research update summarizes the current knowledge about CGs as new, targeted anti-cancer agents, alone or in combination with other chemotherapeutic compounds. Copyright © 2016 Elsevier Inc. All rights reserved.