In normal dividing tissues, cell homeostasis is maintained by rare cellular elements, the stem cells, that have the unique property of self-renewal and differentiation to generate a population of functionally mature tissue elements. Recent studies carried out in the last three decades support the existence of stem cells also in tumors, the so-called cancer stem cells. Cancer stem cells have the property of initiating and maintaining tumor growth, are able to self-renew and to differentiate, are the main drivers of intra- and inter-tumoral heterogeneity and the main cellular mediators of drug resistance, leading to tumor recurrence and metastasis. Cancer stem cells can be identified in many tumors according to specific immunophenotypic features, but cancer stemness cannot be defined as a fixed property, due to cancer plasticity. For these properties, cancer stem cells represent attractive targets for developing new anti-cancer therapies and there is supporting evidence that the combination of conventional anticancer therapies with drugs targeting cancer stem cells could lead to cancer eradication. Ongoing studies in some tumors strongly support the clinical utility of developing efficient strategies of cancer stem cell targeting.