Upcoming large scale particle accelerators, such as the Future Circular Collider (FCC), the Compact Linear Collider (CLIC) and the International Linear Collider (ILC) are expected to require RF drive on the order of 100 MW. Therefore, efforts to improve the efficiency of the specific RF source is of significant interest to the particle accelerator community. Klystrons are an attractive choice as the RF source, with the current state of the art tubes offering efficiencies up to 70%. The High Efficiency International Klystron Activity (HEIKA) collaboration seeks to improve upon this by considering novel methods of electron bunching. Such methods include the core oscillation method (COM), the bunching-alignment-collection (BAC) method, as well as the use of harmonic cavities. The theory behind these bunching methods will be discussed, along with their suitability for specific particle accelerators. In addition, results from numerical simulations predicting klystrons with efficiencies larger than 80% will be presented. Early experimental testing of tubes employing the BAC method will also be presented, demonstrating the efficiency improvements that the scheme offers.