Several imaging modalities have been widely applied for the detection of cancer and its pathological activity in combination with probes capable of improving the contrast between healthy and cancerous tissues. Biocompatible polymeric nanoassemblies have been developed for precise detection of malignant tumors by enhancing the selectivity and sensitivity of the imaging. Exploiting the compartmentalized structure of the nanoassemblies advantageously allows delivering both imaging and therapeutic agents for cancer multifunctional imaging and theranostics, i.e., the combination of therapy and diagnosis tool on a single platform. Thus, nanoassemblies have high potential not only for cancer molecular imaging but also for tracing nanoparticles in biological systems, studying their biological pathways, gathering pathological information, monitoring therapeutic effects, and guiding pinpoint therapies. In this review, polymeric nanoassemblies for optical imaging, magnetic resonance imaging, multifunctional imaging, and image-guided therapy, emphasizing their role in cancer diagnosis and theranostics are highlighted.