In this Ph.D. thesis, the nature of actinide bonds has been investigated mainly using density functional theory (DFT). Various theoretical tools for bonding analysis are examined in this work. The first chapter is an introduction to the chemical properties of actinides as well as the basic concepts of theories and tools that are used in the studies of this thesis. In Chapter 2, the chemistry of uranyl “Pacman” complexes have been investigated theoretically and compared to experimental work from collaborators. The theoretical methods from these projects are the foundation of the theoretical predictions in the rest of the chapters. In Chapter 3, a series of extractants of the polypyridyl family were studied, using DFT, to identify the electronic properties that preferentially influence actinide/lanthanide separations. Various theoretical tools are used to investigate the covalency of actinide bonds. The subsequent experiments by our collaborators confirmed the theoretical predictions. Because the π interactions between extractions and actinides are proved to be important, Chapter 4 further extended the investigation on the π systems of the extractants. In Chapter 5, the solvation effect at high temperature and pressure has been investigated using the change of dielectric constants as a function of temperature and pressure. The results agree well with the experimental data. This provides a convenient model to simulate the actinide aqueous system under extreme conditions that lack experimental data. Chapter 6 investigated the uranyl reduction on the TiO2 surface. We propose that photoreduction can also be triggered by radiation from uranium decay. A new TiO2 cluster was introduced because it is more crystal-like and provides both edges and surfaces. The calculations show that the reduction to uranium IV is more likely to occur on the cluster’s edges. Finally, Chapter 7 summarizes the work in this thesis and provides an outlook on potential future projects. This thesis provides valuable information for understanding the nature of actinide bonds computationally.