  Although the first millisecond pulsars (MSPs) were discovered 30 years ago we
still do not understand all details of their formation process. Here, we
present new results from Tauris, Langer & Kramer (2012) on the recycling
scenario leading to radio MSPs with helium or carbon-oxygen white dwarf
companions via evolution of low- and intermediate mass X-ray binaries (LMXBs,
IMXBs). We discuss the location of the spin-up line in the (P,Pdot)-diagram and
estimate the amount of accreted mass needed to obtain a given spin period and
compare with observations. Finally, we constrain the true ages of observed
recycled pulsars via calculated isochrones in the (P,Pdot)-diagram.
