The monumental discovery of the Higgs boson at the LHC marked the beginning of a new era in the high energy physics. Although the particle spectrum of the Standard Model is now complete with the Higgs boson, the hierarchy problem and the lack of explanation of the origin of dark matter imply that a new Beyond the Standard Model physics should exist. There is however no clear indication (experimental or otherwise) of the energy scale at which this new physics should appear. Current results from the LHC experiments have shown no unpredicted effects up to pp collision energies of 13 TeV. If not observed directly at the LHC, the new physics may reveal itself through deviations of Higgs properties from their Standard Model expectations, or it may become directly accessible only at new, higher-energy accelerator facilities. It is then of primary importance to have a comprehensive review of the available and planned accelerators and their design, physics motivation and expected performance. This book comprises 26 carefully edited articles with well-referenced and up-to-date material written by many of the leading experts. These articles -- originated from presentations and dialogues at the second HKUST Institute for Advanced Study Program on High Energy Physics -- are organized into three aspects, Theory, Accelerator, and Experiment, focusing on in-depth analyses and technical aspects that are essential for the developments and expectations for the future high energy physics.