The growth of GaAs nanowires by the gallium-assisted method with molecular beam epitaxy (MBE) is presented in this review article. The structure of the grown nanowires was investigated by means of scanning and transmission electron microscopy as well as Raman spectroscopy. Their optical properties were revealed by performing photoluminescence measurements at the single nanowire level. Furthermore, by tuning the MBE conditions to planar growth, quantum heterostructures on the side facets of the nanowires were achieved. High-resolution transmission electron microscopy proved that the grown heterostructures have epitaxial precision, while photoluminescence measurements showed that they possess excellent optical quality. These quantum heterostructures constitute templates for developing novel nanowire based devices, such as a high electron mobility one-dimensional transistor or third generation solar cells.