Due to the rapid attenuation of electromagnetic waves in water, positioning and navigation methods in terrestrial network, which are based on electromagnetic wave communication, are not applicable in underwater scenarios. Moreover, the underwater environment is complex and unknown, so underwater navigation and localization are more problematic and difficult. AUV equipped with Strapdown Inertial Navigation System can rely on the information output from the inertial navigation system to perform preliminary navigation underwater, but the inertial guidance has the defect of gradual accumulation of navigation error. To overcome this problem, underwater acoustic positioning is often used to assist AUV positioning and navigation. Underwater acoustic positioning based on long baselines has relatively higher positioning accuracy, wider coverage, and longer positioning distances. Consequently, it emerges as a more fitting choice for deep-sea positioning and navigation. In practical scenarios, a combination of two or more navigation methods are commonly used for integrated navigation. The integration of navigation information derived from diverse navigation methods presents a formidable challenge. Currently, the Kalman filter stands as the most commonly employed filtering method in the fusion of multi-source information, widely applied in the field of navigation. Therefore, this chapter introduces the fundamental principles of Strapdown Inertial Navigation Systems, long baseline positioning, and navigation information fusion method based on Kalman filtering.