Considerable progress has been made in the last several years in the fields of investigation and understanding of the influence of encapsulated substances on the electronic properties of single walled carbon nanotubes (SWCNTs). Relevant data on the modified electronic properties of filled SWCNTs were obtained. The possibility of achieving both acceptor and donor doping and precise changes of the SWCNT doping level by the filling of channels and transformation of incorporated substances was demonstrated. This article presents a comprehensive review of the current status of the research on the electronic properties of filled SWCNTs. The review begins with a brief description of basic aspects of the band theory of solids and peculiarities of the band structure and electronic properties of SWCNTs. The next part of the review is dedicated to a systematization and description of different methods for modification of the SWCNT electronic properties. Then, the review introduces filling methods of SWCNT inner channels. The main part of the review is dedicated to an analysis, systematization and generalization of the up-to-date reported, results on experimental and theoretical investigations of the electronic properties of filled SWCNTs and nanostructures obtained as result of chemical reactions inside the SWCNT channels. Finally, the possible applications of filled nanotubes are highlighted. (C) 2015 Elsevier Ltd. All rights reserved.