Radio spectrum resource demand has increased extraordinarily due to emerging broadband wireless applications which have resulted to critical spectrum shortage problem.  Cognitive radio technology is promising technology that can effectively use unutilized licensed spectrum and can solve spectrum shortage problem. Spectrum sensing is the key element of cognitive radio network to find unused spectrum. Hence effective and accurate spectrum sensing is compulsory for cognitive radio network. This paper is a survey of various advanced spectrum sensing techniques. This paper covers basics of spectrum sensing along with its classification and challenges of spectrum sensing.