Photodetectors are semiconductor devices capable of converting optical signals into electrical signals. There is a wide range of applications for photodetectors such as fiber optics communication, infrared heat camera sensors, as well as in medical and military equipment.Nanowires are thin needle-shaped structures consisting of semiconductor materials such as gallium arsenide (GaAs), indium phosphide (InP) or silicon (Si). They are ideally suited for sensitive photodetectors with low noise due to their small size, well-controlled crystal structure, and composition tunability, as well as the possibility to fabricate them monolithically on silicon.In this thesis, Fourier Transform Infrared (FTIR) Spectroscopy was used to investigate the optical characteristics of InP nanowire-based n+-i-n+ photodetectors with 20 embedded InAsP quantum discs in each InP nanowire. The spectrally resolved photocurrent was measured and analyzed at different angles of incidence. Also, detailed current-voltage characteristics in dark and under illumination were recorded and analyzed.Summarized, the samples showed very good I-V performance with low dark leakage currents. The photocurrent scales with the numbers of nanowires, from which we conclude that most of the photocurrent is generated in the nanowires. Spectrally resolved photocurrent data, recorded at room-temperature, shows strong absorption in the near-infrared region with interesting peaks that reveal, the underlying optical processes in the substrate and nanowires.