I review the chemical abundance measurements of blue horizontal branch stars within the theoretical framework of diffusion and radiative levitation. The present observational scenario reports normal abundances for stars cooler than Teff ~ 11 000 K, while largely anomalous abundances for hotter stars are found. In particular, large He depletions and overabundances of Fe, Ti, Cr, P and Mn are present. On the contrary, the abundances of Mg, Si and Ca are roughly normal, in the hot stars as well as in the cooler ones. Related physical properties as stellar rotation will be also described.