AbstractAlthough 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 PṖ–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 PṖ–diagram.