Nanotubular and nanowire structures of silicon are currently of great interest for miniature devices. Recently, using cluster assembly approach, nanotubular forms of silicon have been shown to be stabilized by encapsulation of metal atoms. This chapter reviews these developments and discusses the stability of such nanostructures and their electronic properties including metallic, semiconducting and magnetic behaviors. Hydrogenated and oxygenated structures of silicon can also be made in tubular forms. These could be among the thinnest semiconducting nanostructures of silicon. Thicker quasi-one-dimensional structures of silicon have been grown in the form of nanowires which could be metallic or semiconducting. This chapter discusses the surface reconstruction in such nanowires and their electronic properties. Further effects of p- or n-type doping as well as hydrogen defects on the atomic and electronic structures of hydrogenated Si nanowires are presented. The metallic, semiconducting and optical properties of silicon in such nanostructures could make it possible to develop novel silicon-based nanodevices. {\textcopyright} 2008 Elsevier Ltd All rights reserved.