  Theoretical aspects of transversity observables are reviewed. The main focus
is on two leading twist transversity single spin asymmetries, one arising from
the Collins effect and one from the interference fragmentation functions.
Electron-positron annihilation experiments which are required to obtain these
fragmentation functions are discussed, as well as the issues of factorization,
evolution and Sudakov factors for the relevant observables. These theoretical
considerations pinpoint the most realistic scenarios towards measurements of
transversity.
