In present days the wireless communication has developed quickly. This demand on wireless application has put several limitations on the existing radio spectrum which is precious and limited. Various frequency bands are not used properly in the fixed spectrum assignment but, the cognitive radio more helpful to utilize these frequency bands which are called as white spaces. This is an exclusive approach to improve use of radio electromagnetic spectrum. There are four methods to establish the cognitive radio. This report explain the first method i.e. spectrum sensing and describe the issues, techniques and challenges of this method.