The need for efficient usage of the scarce electromagnetic spectrum has resulted in the growth of cognitive radios (CRs). The primary task in any CR network is to dynamically sense the radio spectrum so as to reliably determine portion(s) of the frequency band that may be used for communication link(s). Recently spectrum sensing for CRs based on filter banks have been suggested in literature. It was suggested that using filter banks for spectrum sensing will enable multi-carrier communications at no extra cost. This paper presents a study of the requirements and challenges of digital filter banks from a CR perspective in contrast to traditional filter banks for wireless communications. The current state-ofthe- art filter banks are discussed, with both advantageous and disadvantageous and how such filter banks can be altered to meet the essential requirements of CR systems.