We use the notion of covering sequence, introduced by C. Carlet and Y. Tarannikov, to give a simple characterization of bent functions. We extend it into a characterization of plateaued functions (that is bent and three-valued functions). After recalling why the class of plateaued functions provides good candidates to be used in cryptosystems, we study the known families of plateaued functions and their drawbacks. We show in particular that the class given as new by Zhang and Zheng is in fact a subclass of Maiorana-McFarland’s class. We introduce a new class of plateaued functions and prove its good cryptographic properties. Keywords : Boolean functions, Bent, Three-valued crosscorrelation, Nonlinearity, Resiliency, Stream Ciphers, Combinatorial Cryptography.