© 2015 John Wiley & Sons, Ltd. All rights reserved. In this chapter we first briefly introduce Maxwell's equations and derive the Helmholtz equation, a special case of the wave equation, and introduce its different solutions for fields that may propagate in THz waveguides. The second section describes different waveguides operating at THz frequencies. The third section is devoted to the beam waveguide and quasi-optics. Material issues related to waveguides and quasi-optical components are also discussed. The chapter concludes with THz wave propagation in free space.