Two surgical treatments exist for ankle osteoarthritis (AO): ankle arthrodesis (AA) and total ankle replacement (TAR). Due to advantages and drawbacks of each treatment, decision making about these treatments is still a clinical question and thus, their outcome evaluation is of clinical interest. So far, outcome evaluation has been done via clinical scales based on questionnaires and clinical observations. However, clinical scales are subjective and are not accurate enough to detect subtle alterations in foot function. Therefore, objective outcome tools such as instrumented gait analysis are advantageous for evaluation of ankle osteoarthritis and its surgical treatments. In particular, possible alterations of kinematics and kinetics of ankle and other foot joints during gait can be relevant for outcome evaluation of these treatments. On the other hand, gait analysis has been performed in gait laboratories using complex and expensive equipments which are not available in all clinics. Besides, limited measurement space of gait laboratories can perturb the natural gait. Therefore, there is a need for in-field gait analysis for outcome evaluation of ankle osteoarthritis treatments. The aim of this thesis is to design and validate new ambulatory systems for kinematics and kinetics assessment of multi-segment foot during long-distance gait and to apply these systems for objective outcome evaluation of ankle osteoarthritis treatments. The present study proposed objective kinematic criteria for foot segmentation. According to these criteria, for clinical evaluations, foot and ankle complex could be reliably divided into shank, hindfoot, medial forefoot, lateral forefoot and toes. Then, in order to choose an appropriate joint kinematics descriptor, the common mathematical tools to describe the kinematics of foot joints were evaluated and showed similarly high consistency in their results and enough low sensitivity to measurements errors. New algorithms were introduced to estimate rotation of foot joints, 3D ground reaction force and kinetics of foot joints (force, moment and power) using ambulatory systems. First, kinematics of multi-segment foot was assessed, based on body-fixed inertial sensors (3D gyroscope ad 3D accelerometer) on shank, hindfoot, forefoot and toes. Second, plantar pressure insole, as an available ambulatory system in clinics, was used for assessing 3D ground reaction force. An optimal algorithm was proposed for estimating the 3D ground reaction force based on plantar pressure distribution. Third, inertial sensors and pressure insole were used for ambulatory assessment of multi-segment foot joints kinetics. These designed systems were validated with gold standard reference systems. As an advantage, they can be easily used in different clinical environments for long-distance field measurement without perturbing the natural gait. After technical validation of the designed measurement systems, their suitability and efficiency for clinical evaluations were investigated within a clinical protocol. Four populations were involved in the study: patients with AO, patients with TAR, patients with AA and healthy subjects. Gait parameters were measured by ambulatory systems in long-distance walking and were compared between patient groups and healthy subjects. In general, kinetics of foot joints and pressure parameters showed alteration of foot function after AO and were not completely restored after both treatments. Rotation of joints showed improvement only after TAR. Spatio-temporal parameters of gait and their variability showed both of the mentioned effects as well as improvements after both TAR and AA. Therefore, the proposed parameters could detect the differences between patients and healthy subjects before and after both treatments. These parameters also showed clinical relevance based on their correlation with clinical scales. In this thesis, using an ambulatory system consisted of inertial sensors and plantar pressure insoles, a new way of gait analysis to evaluate the function of the foot and ankle complex was proposed and validated. The results provided pertinent metrics for objective outcome evaluation of ankle osteoarthritis treatments and confirmed the suitability of this system for clinical routine uses.