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With increasing demand to support high-data- rate applications, spectral efficiency is becoming more and more important. To exploit limited spectrum efficiently, techniques, such as cognitive radio, device-to-device communications and heterogeneous networks, have been proposed. They make wireless networks more agile and flexible by allowing users/systems to coexist as a result of careful and dynamic planning. In this article, we provide an overview of opportunistic transmission schemes for the scenarios where some existing users/systems have to be protected while new ones can transmit. Schemes exploiting opportunities on frequency and spatial domains will be investigated. Some practical considerations, potential applications and future research topics will also be discussed.