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The electromagnetic spectrum, from 2 MHz to 94 GHz, is an evolving resource. Many uses of the spectrum include communications, radio and television broadcasting, radio navigation, sensing and radar. The region of the spectrum from 2-4 GHz has become particularly problematical due to the influx of wireless systems such as WiMAX and LTE into an area which has traditionally been associated with radar systems. Although the entire EM spectrum is becoming increasingly congested, we will focus our examples on this band as a surrogate for understanding what is happening across the spectrum, while pointing out unique requirements in other bands. This paper will discuss measurements employed for characterizing both radar and non-radar users of this spectrum, as each have unique issues to be addressed. First, the propagation physics of why spectrum users seek access to selected frequency bands. The unique spectral characteristics of radars, not generally found in communications systems will be discussed. The trade-offs in using conventional superheterodyne and FFT based spectrum analyzers versus the newer real time spectrum analyzers, as well as other measurement techniques will be presented in terms of their capability in assessing the EM spectrum and determining how to best use a finite resource. It is projected that many radiating systems will be forced to monitor the EM spectrum and adjust their operating characteristics on a real-time basis. Finally, we will conclude with a short overview of the topics covered in the special session that this paper is a part of.