  Photometrically and spectroscopically variable chemically peculiar (CP) stars
are the optimum laboratories for testing the rotational evolution of main
sequence stars. A vast majority of well-studied CP stars have quite steady
rotational periods (e.g. SrCrEu star CQ UMa). However, there are several CP
stars that exhibit apparent period variations. The origin of the period
variations is unclear in many cases. We describe the observed period variations
of several individual CP stars, especially V901 Ori, sigma Ori E, HR 7355, CU
Vir, SX Ari, and EE Dra. The CP stars with unsteady periods now represent a
very diverse group with dissimilar O-C diagrams and time scales. We also
discuss the causes of the period changes found and a possible cyclicity or
chaoticity of them.
