Ion channels are pore-forming transmembrane proteins that allow ions to permeate biological membranes. Pore structure plays a crucial role in determining the ion permeation and selectivity properties of particular channels. In the past few decades, efforts have been undertaken to identify key elements of the pore regions of different classes of ion channels. In this review, we summarize current knowledge about permeation and selectivity of channel proteins from the transient receptor potential (TRP) superfamily. Whereas all TRP channels are permeable for cations, only two TRP channels are impermeable for Ca2+ (TRPM4, TRPM5), and two others are highly Ca2+ permeable (TRPV5, TRPV6). Despite the great advances in the TRP channel field during the past decade, only a limited number of reports have dealt with functional characterization of pore properties, biophysical aspects of cation permeation, or description of pore structures of TRP channels. This review gives an overview of available experimental and theoretical data and discusses the functional impact of pore-structure modifications on TRP channel properties.