Steganalysis is the art and science of detecting hidden information. Modern digital steganography has created techniques to embed information near invisibly into digital media. This chapter explores the idea of exploiting the noise-like qualities of steganography. In particular, the art of steganalysis can be defined as detecting and/or removing a very particular type of noise. This chapter first reviews a series of steganalysis techniques including blind steganalysis and targeted steganalysis methods, and highlights how clean image estimation is vital to these techniques. Each technique either implicitly or explicitly uses a clean image model to begin the process of detection. This chapter includes a review of advanced methods of clean image estimation for use in steganalysis. From these ideas of clean image estimation, the problems faced by the passive warden can be posed in a systematic way. This chapter is concluded with a discussion of the future of passive warden steganalysis.