The meaning of the title of this review is two-fold; on the one hand

it reflects the general trend towards higher and higher frequencies in the development of modern electronics

particularly the effort made in spanning the microwave infrared gap

and on the other hand

it implies the union of microwaves and optics

which yields Microwave Optics as an interdisciplinary field of study. The paper includes the following sub-titles:1.The early union of microwaves and optice

and its later development: （1） The initial thought of microwave optics; （2 ） The development of microwave optics.2.Spanning the spectrum from microwaves to optics:（ 1 ） The microwaveinfrared gap; （2）The breakthrough of gyrotron and the outgrowth of high-energy or relativistic electronics.3.Microwave optics and the modern optical fiber theories:（ 1 ） Modal theory vs ray theory! （ 2 ） Single-mode and multimode fibers; （ 3 ） Mode couplings; （ 4 ） Measurable quantities; （ 5 ） Quantum theory and formal quantum theory.The author endeavours to describe the evolution of the physical ideas relevant to the variety of problems discussed

and presents some points of view hitherto not explicitely expressed

or not sufficiently emphasized

in the literature.