Silicon and other inorganic semiconductor nanowires (NWs) have been extensively investigated in the last two decades for constructing high‐performance nanoelectronics, sensors, and optoelectronics. For many of these applications, these tiny building blocks have to be integrated into the existing planar electronic platform, where precise location, orientation, and layout controls are indispensable. In the advent of More‐than‐Moore's era, there are also emerging demands for a programmable growth engineering of the geometry, composition, and line‐shape of NWs on planar or out‐of‐plane 3D sidewall surfaces. Here, the critical technologies established for synthesis, transferring, and assembly of NWs upon planar surface are examined; then, the recent progress of in‐plane growth of horizontal NWs directly upon crystalline or patterned substrates, constrained by using nanochannels, an epitaxial interface, or amorphous thin film precursors is discussed. Finally, the unique capabilities of planar growth of NWs in achieving precise guided growth control, programmable geometry, composition, and line‐shape engineering are reviewed, followed by their latest device applications in building high‐performance field‐effect transistors, photodetectors, stretchable electronics, and 3D stacked‐channel integration.