A short overview is given on recent polarized light microscopy studies of the orthorhombic ferroelastic domains and domain structure of YBa2Cu3O7-x. The optical characteristics of large domains and lamellar domains with a high density of twin walls are pointed out, both for reflected and transmitted light. The polarizing microscope has allowed to observe the ferroelastic detwinning in situ and to determine the activation energy of the ferroelastic wall movements. The question of a potential ferroelectric phase transition is also discussed.