Identification of the driver mutations in cancer has resulted in the development of a new category of molecularly targeted anti-cancer drugs. However, as was the case with conventional chemotherapies, the effectiveness of these drugs is limited by the emergence of drug-resistant variants. While most cancer therapies are given in combinations that are designed to avoid drug resistance, we discuss here therapeutic approaches that take advantage of the changes in cancer cells that arise upon development of drug resistance. This approach is based on notion that drug resistance comes at a fitness cost to the cancer cell that can be exploited for therapeutic benefit. We discuss the development of sequential drug therapies in which the first therapy is not given with curative intent, but to induce a major new sensitivity that can be targeted with a second drug that selectively targets the acquired vulnerability. This concept of collateral sensitivity has hitherto not been used on a large scale in the clinic and holds great promise for future cancer therapy.