In this talk, I am aiming to cover several important aspects of opportunistic (challenged) networking research, mainly from the system and practice approach. First, I would emphasize that system research can be fun and understanding of the underlying real environment and the need of local people is very important for researchers to tackle the real problem.  In this regard, I will share my experience on deploying challenged networks into several rural areas in the past 4 years, including the Swedish Lapland, India Himalaya, Brazilian Amazon, and Swiss Alps.  Mobility is a double-edge sword; it creates disruption and disconnection in the network, but at the same time potentially increases the capacity of the network.  Understanding mobility is a key to opportunistic networking research. I will introduce several mobility experiments using mobile devices and illustrate several observed properties regarding contact patterns and social behavior, which are shown to have high impact on forwarding protocol performance. Third , I will highlight how planet-scale public web camera can be exploited to explore mobility and dynamics of global cities. On the other hand, the feasibility of opportunistic communication largely rely on the willingness of potential relays to forward information for others, and for that matter, altruism is an important factor. Fourth, I will outline an experiment for measuring the altruism level of mobile users, discuss the challenges, and show some results regarding the impact of battery level, social ties, and mood.  A large scale and realistic testbed is essential for performance evaluation of challenged networks. Finally,  Killer applications are the key to put theory into reality and to ensure the sustainability of the research field. I will select several potential examples, which include using opportunistic networks for cellular traffic offloading, privacy-preserved adverts, and anonymous communication in the presence of adversary authorities.