Book excerpt: Recent advances in miniature sensors and mobile computing have fostered a dramatic growth of interest for 'ambulatory' human motion tracking. Inertial (i.e. accelerometers and gyroscopes) and magnetic sensors do not have in-the-lab measurement limitations and thus are ideal for ambulatory applications. This thesis presents ambulatory human motion tracking using inertial/magnetic sensing. In particular, the purpose of this thesis is to introduce novel orientation estimation algorithms using an inertial/magnetic sensor and demonstrate practical applications of the inertial/magnetic sensors in spinal and gait analysis. First, two quaternion-based orientation estimation algorithms were newly developed with focus on improving computational efficiency. Both algorithms deal with so-called Wahba's problem, a least squares minimization problem, to find a best fit orientation estimation solution. A major difference between them is that one is based on a deterministic approach using a Gauss-Newton method and the other is based on a stochastic approach that employs Kalman filtering. The Gauss-Newton method in the former was formulated using virtual rotation concept while the Kalman filter in the latter was designed to have a minimum-order structure, which significantly improves the computational efficiency of each algorithm. Second, a novel 3D spinal motion measurement system based on inertial/magnetic sensors was proposed. The proposed system can provide not only 3D orientations of the spine/pelvis but also temporal gait parameters, enabling a comprehensive analysis of the 3D spinal kinematics together with the gait analysis. In particular, the spinal motions during the staircase walking were compared to those during level walking using the proposed system, to fill a gap in the spinal kinematics literature. Furthermore, the system was applied to investigate low back pain effects on spinal motion during stair-climbing. This study revealed that the lumbar spinal sagittal motion during stair-climbing can provide an effective quantitative measure in the assessment of low back pain patients. In addition to the spinal motion analysis, an automatic gait event detection algorithm using shank attached inertial sensors was presented for further gait analysis. The outcomes of the research in this thesis can serve as foundation towards achieving a truly ambulatory human motion tracking system.