2D transition metal dichalcogenides (TMDs) are promising nano-electronic materials. The ability to dope 2D TMDs both n- and p-type with high carrier concentration and mobility is essential to the development of MOSFET and CMOS technologies. WSe₂ has been shown to exhibit both p- and n-type conductivities after chemical doping; however, the n-type doping is much less efficient than the p-type doping. We have carried out density functional theory calculations of a wide range of transition metal dopants in WSe₂ with the goal to understand the different dopant behaviors and to design new n-type dopants with improved doping efficiency. We have also studied in details the native defect properties since defects can potentially compensate the dopants. The calculated trend of dopant formation energies is consistent with the available experimental results. We will discuss the underlying mechanisms that are responsible for the different dopant formation energies based on the local symmetries of the dopants in WSe₂ and in competing secondary phases, which affect the crystal field splitting and the relative chemical stability of dopants in different crystal environments. We will also suggest new dopants based on our calculations.