The intent of this monograph is to provide a comprehensive review on manganese dioxide (MnO2) particles, to guide the readers on the process involved from synthesis, through optimisation of functional properties to an overview of some exciting applications. MnO2 is a narrow bandgap semiconductor material which is theoretically responsive to visible light. This monograph discusses the fundamental aspect of MnO2 particles such as crystal structure, synthesis techniques, photocatalytic mechanisms for organic dyes and silver metal ions removal. It also reviews the factors that are affecting the performance of MnO2 particles in removing organic dyes and silver metal ions. In addition, it presents the other applications of MnO2 such as in rechargeable battery, supercapicator and biosensor. Last but not least, this monograph also discusses the challenges and future perspectives of MnO2 particles. This monograph could be a good reference for researchers who are new in the works on MnO particles as it is relatively simple and clear without involving in-depth theoretical derivation.