Research on mesogens and mesophases began with the surprising observat
ion of a fluid and birefringent phase formed by a pure compound having
a calamitic (rod-like) molecule. The structures of the smectic A, nem
atic and cholesteric phases which emerged after this discovery were el
ucidated essentially by observations with a polarizing microscope. Sme
ctic polymorphism was also established by microscopic observations. Th
e main historical results on the structures of the different smectic m
esophases are listed here. The discotic mesophase, previously predicte
d theoretically, was then obtained and its polymorphism observed. A fu
rther type of purposely designed mesophase was the pyramidic type, als
o exhibiting a rich polymorphism. Research on mesogens and mesophases
is a field in continuous expansion.