Nanostructures have attracted substantial attention due to their distinctive properties and various applications. Nanostructures consisting of multiple morphologies and/or materials have recently become the focus of intense study with particular attention being paid to their optical and magnetic properties and the enhanced role of the interface between materials. Of particular interest are metallic-based plasmonic nanostructures, structures that support surface plasmon resonances that are sensitive to the environment, and ferrimagnetic-based nanostructures, structures that exhibit strong magnetic properties when exposed to an external field. These nanostructures provide theranostic potential in the context of cancer photothermal therapies, diagnostics and imaging. Additionally, chalcogenide based nanostructure complexes are particularly interesting. Metallic chalcogenides offer the ability to combine different types of linear and nonlinear optical properties, enable design of nanostructure complexes with surface plasmon resonance effects in new wavelength ranges, and act as photo-emitting agents for novel theranostic applications. In this thesis an in depth analysis of plasmonic, magnetic and photo-emitting nanostructures as theranostic agents is presented. We have created several multifunctional nanostructures and the factors contributing to the functional properties of these nanostructures are explored systematically through experimentation, theory, and simulations. Both in vivo and in vitro testing demonstrates the applicability of these nanostructures as theranostic agents.