The Sun as now studied is far removed from its early models that described a homogeneous spherical body: chemical fractionation as well as considerable fine-structure in and above the solar surface and internal mixing must now be taken into account. Accordingly, attention in composition studies has shifted from absolute solar abundances toward differences in the chemical (and isotopic) composition of the different parts of the Sun: in its interior, in the structures that make up its atmosphere, and in solar-wind and other, more energetic, particle streams. As was the case with the earlier photospheric composition studies, the investigation of abundance differences results in further clues and input for understanding, in particular, mass-transport, separation and heating mechanisms. This volume contains the reviews and contributions on these topics presented at an ISSI Workshop, and it summarises the resulting discussions in rapporteur papers. The book is intended to provide both active researchers and advanced graduate students with a comprehensive up-to-date `compte rendu' on solar composition.