The Armed Forces' access to adequate frequency spectrum is vital for functioning radio networks and the command and control capability. The frequency spectrum is limited and subject to competition from commercial interests, which limits the Armed Forces' exclusive access to frequency spectrum. With the growth of the Armed Forces, the communication demands increase and thus access to frequency spectrum increases. The purpose of this report is to increase the knowledge of techniques used in wireless communication systems for flexible spectrum use. The aim is to present techniques that allow flexibility in the way the frequency spectrum is used by wireless communication systems and challenges of using such technologies in military radio systems. The survey is based on open sources such as articles from scientific journals and standards for wireless communication systems. The scope of civilian research in this area is extensive, so a complete survey is not realistic, but parts that we have found relevant are presented. Research in this field has increasingly utilized machine learning to solve these problems in recent years. One trend is multi-functional systems, that is radio-systems with additional functions other than communication, for example radar applications, jamming or signal intelligence. There are two standards, CBRS in the US and LSA in Europe, for sharing of the frequency spectrum. In cellular systems, there are several techniques to manage the frequency spectrum flexibly to increase data rates for the user. Fundamental differences between civilian communication systems and military tactical radio systems exist, for example, the civilian systems are often centralized and standardized while military systems are decentralized and rarely standardized, which affect the conditions for flexible frequency use. The initial step for the Armed Forces is to broaden the sharing of the frequency spectrum that is exclusively allocated to them. Current tactical radio systems have limited automatic capabilities, so manual methods are necessary. In the long term, new radio systems will be required to have the capability to share spectrum.