Isolated through combinatorial libraries by an iterative in vitro selection process, small single-stranded nucleic acid compounds, named aptamers, have been developed as high-affinity ligands for a variety of targets, ranging from small chemical compounds to large proteins. In the last years, an increasing number of aptamers has been generated that represent potential antagonists of the disease-associated target proteins. These molecules have been shown to discriminate between even closely related targets, thus representing a valid alternative to antibodies or other biomimetic receptors for the development of biosensors and other bioanalytical methods. Moreover, they can be easily stabilized by chemical modifications for in vivo applications and numerous examples have shown that stabilized aptamers against extracellular targets such as growth factors, receptors, hormones, or coagulation factors are very effective inhibitors of the corresponding protein function, thus resulting as useful reagents for target validation in a variety of diseases, including cancer. Indeed, many signaling proteins involved in diverse functions such as cell growth and differentiation can act as oncogenes and cause cellular transformation, thus making these high affinity ligands promising tools for cancer diagnosis or therapy.