This chapter provides overview of fundamental definitions, tools and new methods towards improved channel modeling reported in the Co-operation in Science and Technology (COST)-Inclusive Radio Communications (IRACON) Action for future wireless communications and networks. The overview first covers definitions of propagation environments as they determine most relevant propagation mechanisms to consider and model, and furthermore, guide approach to channel modeling methods. This chapter then introduces new insights into popular approaches of channel modeling, i.e., site-specific and geometry-based stochastic channel modeling, where the latter particularly features canonical and standardized channel modeling approaches taken by the 3rd Generation Partnership Project (3GPP), COST, and International Telecommunication Union (ITU) communities. Finally, this chapter shed lights on new modeling approaches to small-scale radio propagation behaviors, covering plane wave propagation paths and distributed diffuse scattering.
