Cancer remains a leading cause of global morbidity and mortality, with its increasing prevalence driven by genetic, environmental, and socio-economic factors. Traditional therapies such as chemotherapy and radiotherapy often suffer from non-specific targeting, severe side effects, and multidrug resistance. In response, theranostics, an integrated approach combining diagnostics and therapeutics, has emerged as a promising alternative. Nanoparticle-based theranostic systems, with their unique physicochemical properties, offer significant advantages by enabling the simultaneous delivery of diagnostic and therapeutic agents. Nanomaterials like gold, zinc oxide, iron oxide, silver, silica, quantum dots, and hybrid nanoparticles have shown great potential in improving cancer care by enhancing drug targeting, reducing toxicity, and enabling advanced imaging techniques. Gold nanoparticles, for instance, exploit surface plasmon resonance for imaging and photothermal therapy, while zinc oxide nanoparticles generate reactive oxygen species to induce cancer cell apoptosis. Iron oxide nanoparticles combine magnetic resonance imaging with magnetic hyperthermia therapy. Despite promising preclinical results, challenges such as toxicity, scalability, and regulatory issues persist. This chapter explores the diverse applications of nanomaterials in cancer theranostics, discussing their potential to improve diagnostic accuracy, therapeutic efficacy, and patient outcomes, and highlights the need for further research to overcome existing barriers to clinical translation.