Thanks to the single photon sensitivity and excellent surface-to-surface resolution, single-photon LiDAR has emerged as a prime candidate technology for depth imaging through challenging environments. In this talk, we will first introduce the concept and principle of single photon imaging, discussing the benefits, progresses, as well as the applications of such technique. Then, we shall discuss those problems that can not be solved by hardware alone (e.g. sparse 3D data, strong ambient light, multiple returns), which is considered as a barrier to widespread deployment in a number of applications (e.g. long-range imaging, imaging through turbid media, sensing of complex environment). Furthermore, as the above problems motivated the design of photon efficient algorithms, we shall present the iconic ideas and solutions that achieved state-of-the-art results, bring together the innovative thoughts and prospect the future development and challenges in this field.