The synthesis of nanostructured ZnO has attracted considerable attention worldwide because of its practical and potential applications. Numerous low-dimensional ZnO nanostructures, such as nanotubes, nanowires, nanobelts, nanorods, etc. have been synthesized. Whilst improvements in performance can be observed in such nanomaterials over bulk ZnO, it is expected that hierarchically structured ZnO can increase performances further still as the advantages of nanoscale structuration can be combined with the advantages of micro and macrostructuration on different length scales. However, investigations on synthesis, characterization, and applications of hierarchically nanostructured ZnO are scarce. The objectives of this thesis are to design and synthesize a series of hierarchically structured ZnO, and to evaluate their potential in photocatalytic applications. The scientific strategy for the synthesis of hierarchically structured ZnO is to design suitable synthesis routes based around various zinc sources available. In this thesis, two different types of zinc sources, organic (i.e. dimethylzinc and zinc acetate) and inorganic (i.e. zinc nitrate and zinc chloride), have been employed to synthesize hierarchically structured ZnO. A series of hierarchical ZnO have been successfully achieved by adopting various synthesis methods. The obtained hierarchically structured zinc oxides were characterized in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption-desorption measurements, room-temperature photoluminescence spectroscopy (RT-PL), and thermogravimetry and differential scanning calorimetry (TG-DSC). Based on these characterizations, formation mechanisms were reasonably proposed. Finally, the photocatalytic performances of the as-synthesized hierarchically structured ZnO materials were investigated by examining the photodegradation of Rhodamine B (RhB) under UV light in the presence of these materials. The present work demonstrates that hierarchically structured ZnO could be achieved by designing suitable synthesis routes dependant on the zinc source chosen. This thesis not only provides effective ways to hierarchically structured ZnO materials, but also obtains some beneficial results in terms of the optical and photocatalytic properties of these materials. This scientific strategy could subsequently be expanded to the synthesis of other hierarchically structured metal oxides.