Abstract: Wave-particle interactions can naturally result in energy transfer between electromagnetic fields and particles in collisionless plasmas, and these interactions can therefore reshape the particle velocity distribution function. How to quantify wave-particle interaction is one fundamental problem in the heliosphere and astrophysical plasma communities. Recently, there are many important findings related to the wave-particle interaction. This paper will report recent theoretical developments in the study of the wave-particle interactions. In particular, this paper will introduce a newly-developed theoretical method in quantifying both resonant and nonresonant wave-particle interactions. This paper will also introduce applications of quantifying wave-particle interactions for the Alfvén-mode wave, the proton beam instability, and the electron heat flux instability in the inner heliosphere.