This chapter describes cell-type specific aptamers and their use in diagnostics and therapy. Aptamers are single-stranded oligo(deoxy)nucleotides that selectively bind to their target molecules with high affinity. Cell-type specific aptamers in particular can be identified via SELEX using isolated surface proteins or whole cells as targets.Cell-type specific aptamers have been mostly selected targeting cancer cells, which essentially take into account that about 20 % of the deaths worldwide are due to cancer and cancer-related diseases. In the early stages of cancer, circulating cancer cells are very rare. Cancer cell-targeting aptamers allow the identification of these rare circulating cells, thereby providing new tools for early cancer detection and diagnosis. Furthermore, they can be easily synthesized with a variety of modifications. In this regard tumor cell-targeting aptamers are employed as potential delivery vehicles, whereas they are equipped with various cargo molecules, such as toxins, chemotherapeutics, or siRNA molecules, that allow for the development of cell-specific treatment regimens and the decrease of unwanted side effects. Contents