Imaging plays an important role in disease diagnosis, prognosis, and monitoring of therapeutic responses. Multiple imaging modalities (e.g., ultrasound, CT, MRI, SPECT, and PET) are routinely used in clinical practice, while others (e.g., optical, and photoacoustic imaging) are starting to find niche applications in the clinic, such as intraoperative image-guided procedures. Many of those imaging techniques require contrast agents to enhance the distinction between healthy and pathological tissues. In the last two decades, nanoparticles have been increasingly explored as imaging probes as they pose unique advantages, such as:

1. Some are optically or magnetically imageable, while others can be loaded with dyes or radiotracers.
2. Nanoparticles tend to circulate for prolonged periods and display passive accumulation at pathological sites, including tumors and sites of inflammation.
3. Nanoparticles can be further functionalized with targeting vectors to promote interaction with targeted cells.
4. Because nanoparticles are also extensively used in drug delivery, theranostic nanoparticles (with both therapeutic and imaging features) can be used to better understand their behavior in vivo, providing fundamental insights to enhance the therapeutic formulations.

Since nanoparticles have become increasingly relevant in preclinical and clinical imaging, this Methods Collection will cover the development of nanoparticles as imaging contrast agents. Some topics will include the synthesis and characterization of imageable nanoparticles, strategies to enhance their imaging performance, radiolabeling protocols for their use in nuclear imaging, their functionalization for molecular imaging, and theranostic applications.