Resistance to chemotherapeutic drugs presents a big caveat for cancer treatment. In this review we will describe the molecular mechanisms involved in chemoresistance, discussing the mechanisms of resistance related to tumour microenvironment, as well as their intracellular mechanisms. Chemoresistance can also appear as a consequence to treatments with new anticancer drugs. In this sense, we will exemplify this type of resistance discussing mechanisms of action of epidermal growth factor receptor (EGFR) inhibitors. We conclude that the main problem of chemoresistance is due to its pleiotropic and multifactorial nature. © Springer-VerlagItalia 2007.