The current way to increase efficacy of cancer therapy is the use of molecular recognition of aberrantly expressed gene products for selective treatment. However, only a fraction of the patients have tumors with a particular molecular target. Radionuclide imaging of molecular targets might help to stratify patient for cancer treatment. Affibody molecules are scaffold proteins, which can be selected for high affinity recognition of proteinaceous molecular targets. The capacity to re-fold under physiological conditions allows labeling of Affibody molecules in a broad range of pH and temperatures with preserved binding properties. Peptide synthesis or introduction of a unique cysteine enables sitespecific labeling of Affibody molecules, resulting in uniform conjugates with well-defined pharmacological characteristics. The small size (7 kDa) of Affibody molecules provides rapid extravasation, rapid tumor penetration, and rapid clearance of unbound tracer from healthy organs and tissues. In combination with sub-nanomolar affinity, this results in high contrast in vivo imaging a few hours after injection. Excellent targeting has been demonstrated in pre-clinical studies with HER2-targeting Affibody molecules labeled with 99mTc and 111In for single photon computed tomography (SPECT), and 18F, 64Cu, 124I and 68Ga for positron emission tomography (PET). Pilot clinical data confirm the high potential of Affibody molecules.