Theranostic nanoparticles present a powerful approach to combine therapeutic with diagnostic functions, resulting in an effective means of combating complex diseases, such as cancer. Several aspects need to be taken into account for developing theranostic nano-agents: the imaging unit must provide high contrast, the therapeutic component needs to be effective. Moreover, the nanoparticles should have long circulation times and be able to target the specific site of the disease, and the carrier units should be capable of on-site release and clearance after release of the drug. A wide range of nanoparticles produced from a diverse set of precursors and in a variety of sizes and shapes has been developed to implement diagnostic or therapeutic modalities. To function as a diagnostic imaging nanoparticle, either a molecular contrast agent is incorporated into the material making up the nanoparticle, or the nanoparticle material itself exhibits contrast. Inorganic nanoparticles, such as quantum dots and plasmonic gold nanoparticles, are often brought forward as probes for biomedical imaging. However, their restricted biocompatibility and limited ability to incorporate therapeutic agents or undergo biodegradation impede their theranostic application. Similarly, conjugated polymers, show great promise as diagnostic nanoparticles, due to their superb biocompatibility and inherent contrast either in fluorescence or photoacoustic mode. Yet, there are only few examples, where conjugated polymers and their nanoparticles can be biodegraded. Importantly, biodegradation is a prerequisite for their application in vivo, therefore preventing most semiconducting polymers from being used as theranostic nanoagents.