This paper gives a unified and historical review of observer design for the benefit of practitioners. It is unified in the sense that all observers are examined in terms of: 1) the assumed dynamic structure of the plant; 2) the required information, including the input signals and modeling information of the plant; and 3) the implementation equation of the observer. This allows a practitioner, with a particular observer design problem in mind, to quickly find a suitable solution. The review is historical in the sense that it follows the evolution of ideas in observer design in the last half century. From the distinction in problem formulation, required modeling information and the observer design goal, we can see two schools of thought: one is developed in the framework of modern control theory; the other is based on disturbance estimation, which has been, to some extent, overlooked.