On the main sequence, roughly 10-20% of A- and B-type stars display a wide range of chemical peculiarities in their photospheres. It is unknown when during stellar evolution these chemical peculiarities develop, and the mechanisms for the formation of some peculiarities are Dot fully understood. In order to provide strong observational constraints, this thesis investigates chemical abundances in Herbig Ae and Be (HAeBe) stars, which are pre-main sequence progenitors of A and B stars. A detailed abundance analysis was performed for 20 HAeBe stars, and 1 dusty young star, using high resolution, high signal-to-noise ratio spectra. The abundance analysis proceeded by directly fitting synthetic spectra to observations, determining effective temperature, surface gravity, microturbulence, and projected rotational velocity self-consistently with chemical abundances. The synthetic spectra were computed with the program ZEEMAN, which was developed further, optimising it for nonmagnetic spectrum synthesis, adding an automatic fitting routine, and parallelising the program. Eleven of the stars are found to display 'Boots chemical peculiarities, one star shows weak Ap/Bp peculiarities, and the remaining 9 stars are found to be chemically normal. The star with weak Ap/Bp peculiarities has a confirmed magnetic field detection in the literature, as do one],. Boo star and one chemically normal star. The other stars have been searched for magnetic fields, but have no confirmed detections. I argue that the large incidence of Boo chemical peculiarities among HAeSe stars provides strong evidence in favour of a selective accretion hypothesis for the formation of A Boo peculiarities. Among the magnetic stars, it appears that Ap/Bp peculiarities can form on the pre-main sequence but, unlike the main sequence, there' are also chemically normal magnetic HAeBe stars.