One of the important challenges in the modern physics is the development of all-optical devices enabling efficient generation, transmission, and processing of optical signals, since direct manipulation of optical signal pulses with photonic elements is more efficient compared to approaches involving successive opto-electronic conversions. It is desirable to develop an optical chip integrating multiple photonic elements, however this demands ultra-low optical losses, which is difficult to achieve with mico- and nano-scale structures. Recently it was shown that Parity-Time (PT) symmetric optical systems can offer promising approach to address these challenges. Optical PT systems feature symmetrically distributed regions of gain and loss. Losses here are as important as gain and there is no need to avoid them. Depending on the structural parameters, gain can compensate losses, or signal amplification can be realized. Generally, PT systems demonstrate nontrivial wave interactions and phase transitions, which can be used for signal filtering and switching, opening new prospects for active control of light. Nonlinearity in PT-symmetric systems provides basis for all-optical control and switching. Additionally, PT structures can offer new possibilities for generation and control of quantum photon states.