One-dimensional (1D) semiconductors, or nanowires, have interesting physical properties and may offer possibilities to realize ultra-small electronic and photonic devices@footnote 2@ as well as new kinds of circuitry.@footnote 3@ Well defined nanowires (or nanowhiskers) can be formed using size selected catalytic nanoparticles to induce growth. By locating individual nanoparticles on predefined locations on a substrate also the position of the nucleation of individual nanowires may be controlled.@footnote 4@ It was recently shown that not only homogeneous nanowires, but also multi-heterostructures with good electronic properties may be formed within a nanowire, even for combinations of materials having large differences in their lattice constants.@footnote 5@ I will describe results from our studies of the electronic properties of heterostructures and will present examples of 1D heterostructure nanoelectronic devices created in nanowire systems. @FootnoteText@ @footnote 1@ I want to ackhowledge contributions from a number of students and colleagues (see author lists in refs. 5 and 6), especially Jonas Ohlsson on whisker growth, Mikael Bj@um o@rk on physics investigations and Prof. Reine Wallenberg on transmission el ectron microscopy.@footnote 2@ K. Hiruma et al., "GaAs p-n junction formed in quantum wire crystals", Appl. Phys. Lett. 60, 745 (1992)@footnote 3@ Y. Huang et al., "Directed assembly of one-dimensiona l n anostructures into functional networ ks", Science 291, 630 (2001)@footnote 4@ B. J. Ohlsson et al., "Size-, shape-, and position-controlled GaAs nano-whiskers", Appl. Phys. Lett. 79, 3335 (2001)@footnote 5@ M. T. Bj@um o@rk et al., "One-dimensional steeplechase for electrons realized", Nano Lett. 2, 87 (2002). @footnote *@Lars Mamuleson E-mail: lars.samuelson@ftf.lth.se