Abstarct: The purpose of this thesis is to design an appropriate criterion for bearing-only tracking (BOT) problem in 3D case using the relevant measurements. Since the BOT problem is unobservable, this criterion is meant to have an appropriate estimation with guaranteeing observability of the target. Due to lack of information from the motion range of the target, this tracking problem is a passive tracking that the basis of its work in 3D is to estimate the target paths moving in a noisy environment with constant speed in a straight line. In this project, two important issues are studied: 1) estimation of the target motion parameters including position, velocity, angles and course using batch and recursive algorithms, 2) the analysis of the target observability using observer maneuvers. Finally, three appropriate approaches are proposed considering the problem of observability and estimation of the motion parameters.