This text develops a mathematical and physical theory which takes a proper account of the elasticity of liquids. This leads to systems of partial differential equations of composite type in which some variables are hyperbolic and others elliptic. It turns out that the vorticity is usually the key hyperbolic variable. The relevance of this type of mathematical structure for observed dynamics of viscoelastic motions is evaluated in detail. Much attention was paid to observations - most of which are not older than five years - following the attitude that experiments are the ultimate court of truth for physical theories. Readers will find their understanding of all problems involved highly enriched.