Various methods exist for observing marine mammals, each having its advantages and drawbacks. It is possible to employ more one method simultaneously as they are complementary. This article presents these methods and focuses on the use of passive acoustics for marine mammal observation. We provide specifics on the required material, the various underwater acoustic propagation models and the software used for the detection of sounds emitted by cetaceans. This paper presents the recent international projects included in marine mammal observation. The conclusion will outline perspectives on the passive acoustic methods for whale observation.