Energy conversion systems for renewable energy rely on mechanical gearboxes to transmit mechanical power to the electrical machine at higher speeds and to avoid having very large generators. The main issue with mechanical gears is their low reliability and requirement for regular lubrication and maintenance. Magnetic gears perform power transmission between two ports through the frictionless interaction of magnetic forces. Some potential advantages of using magnetic gears are reduced maintenance, possible isolation between input and output shafts, and inherent overload protection. To cover all the objectives defined, the work was divided into four chapters. The literature overview of the first chapter has shown that remarkably high-performance magnetic gears were developed. In the second chapter, 3D and quasi-3D computationally efficient magnetic equivalent circuit models that could consider the magnetic saturation and the end-effects were presented. It was found that these methods could achieve an important time gain against the finite element method, albeit with a slight reduction in the accuracy. The third chapter included an overview of the marine renewable energy systems and the magnetic gear applications in this field, and then an optimization study was done to propose a magnetic gear design that could be suitable for marine renewable energy applications. The fourth chapter presented a study and analysis on the eccentricity defect that could occur in tubular linear machines and magnetic gears, where a 3D finite element method model was used in the analysis first, and then to reduce the computation time a quasi-3D finite element method tool was developed.