Reliable and unceasing exploitation of power electronic converters plays a major part in every application. This PhD thesis comes up with new opened-up reliability assessment frameworks and demonstrates the feasibility of using multistate degraded system analysis and interval analysis as well for attaining much more accuracy in reliability evaluation. Considering self- and coupling degradation effects of the items and assessing reliability in the system level are important issues which are still lacking in the previous studies and the present thesis has made an effort to overcome these problems. The thesis tries to launch two distinct reliability assessment frameworks, namely interval reliability and Multistate degraded system reliability.Interval reliability is capable of introducing an interval useful lifetime for DC-DC power electronic converters in which the self and coupling degradations effects of items have been taken into account. Fundamentally, it is based on the conventional physics of failure reliability assessment. Based on this method, instead of obtaining an inaccurate reliability value, one can attain an interval for the reliability of a global system whose useful lifetime undoubtedly lays between the boundaries. This method is able to consider degradation dependencies and assess multi-component system level reliability but not redundant system. An attempt was made to enhance the conventional reliability framework to reach a systematic approach capable of estimating a reliability assessment considering self and mutual degradation effects and being able to evaluate system-level reliability including redundant system. Multistate degraded system reliability analysis is capable of estimating system-level reliability, while mission profile and physics of failure of the system’s items are taken into account. In addition, the self and mutual degradation effects of items on the operation of the global system have been considered. The multi-state degraded system reliability model exposed to multiple failure processes has been generalized. The operating condition of the global system is defined by a finite number of states. Not only can the proposed framework be employed in determining the reliability of the degraded systems in terms of multi-state functions, but also it can obtain the states of the systems by estimating the system state probabilities.As an application, a DC-DC power electronic system containing three critical items has been studied. In this case study, two power semiconductors, namely IGBT and diode, and a power capacitor have been considered as three degradation processes and their aging effects on the useful lifetime estimation of the power electronic system have been discussed. For having a sense about newly proposed reliability frameworks, a 3000W and 200/400 V conventional DC-DC converter for electric vehicle application exposed to WLTP driving cycle is considered.