As stated by the WHO, cancer is the second leading cause of death globally, leading to the fact that 1 in 6 deaths is due to cancer. Additionally, the economic impact of cancer is both significant and increasing; hence, visualization as well as effective tailored treatment is crucial and becomes more and more important nowadays. Moreover, early detection of primary tumors, as well as potential metastases, has a significant impact on the therapy regime and overall survival rates of patients. Therefore, both clinician and patient would benefit highly from appropriate (imaging) agents.

As prerequisites, different molecular imaging modalities (PET, SPECT, CT, MRI, ultrasound, and nanoparticles) and corresponding agents have been developed or are in development for the combat against cancer. For example, highly sensitive and specific in vivo agents allow for the visualization of receptor systems, enzymes, and proteins involved in cancer biochemistry and/or pathophysiology. Furthermore, the development of tracers for the characterization of the vast heterogeneity of cell types within a tumor and its microenvironment (stromal cells, endothelial cells, immune cells, etc.) is and will be of great value to tackle some of the so far unanswered questions regarding cancer and its mechanisms.

Although there is evidence for ongoing progress in these fields, one has to admit that research is it still in its infancy. Hence, it is the aim of this special issue to encourage high quality work contributing to these important topics.

Potential topics include but are not limited to the following:

• PET- and SPECT-tracers as specific targets for the clinical visualization and treatment of cancer, cancer cells, and the tumor microenvironment
• MRI agents for the in vivo detection of malignancies
• Imaging agents for the early diagnosis of tumors and metastases
• Emerging tracers for the treatment of cancer
• Nanoparticles for the imaging and treatment of cancer and cell related cancer processes
• Diagnostic, therapeutic, and theranostic in vivo agents with direct clinical application
• Imaging biomarkers for colon, mamma, and prostate carcinoma and melanoma
• Recent advances in preclinical and clinical application of in vivo biomarkers of cancer mechanisms or cancer cells
• Macromolecules for the imaging of cancer and malignant processes