What is the Higgs boson? How are all forces unified in particle physics? What is a mechanism for gauge symmetry breaking? Are there extra dimensions in spacetime?The Higgs boson discovered in 2012 is a crucial piece in the standard model for unifying electromagnetic and weak forces. Gauge symmetry is spontaneously broken by the Higgs mechanism. There is an alternative scenario. In the Hosotani mechanism gauge symmetry is spontaneously broken by dynamics of gauge fields in the fifth dimension. The Higgs boson appears as a fluctuation mode of an Aharonov-Bohm phase in the fifth dimension. This leads to the scenario of gauge-Higgs unification.In this book gauge-Higgs unification is introduced from the basics and is applied to electroweak unification. It is seen that gauge-Higgs unification gives nearly the same phenomenology at low energies as the standard model, and leads to many predictions to be confirmed in future experiments.