This　paper　experimentally　investigates　the　pull-out　behavior　of　single　studs　and　stud　groups　embedded　in　ultrahigh-performance　concrete　(UHPC).　A　total　of　33　pull-out　tests　are　conducted.　The　effects　of　the　fiber　content　(rho),　effective　embedded　depth　of　stud　(hef),　ratio　of　embedded　depth　to　diameter　of　stud　shank　(hef　/d),　diameter　of　stud　head　(dh)　are　considered　for　single　studs;　the　effects　of　the　arrangement,　number,　and　spacing　of　studs　are　considered　for　stud　groups.　Results　indicate　that　when　concrete　failure　governs,　for　single　studs　in　UHPC　with　fibers:　(i)　the　pull-out　strength　and　ductility　increase　with　increasing　rho,　hef,　or　dh,　(ii)　the　pull-out　strength　and　ductility　are　higher　than　those　in　normal　concrete　(NC),　and　(iii)　the　radius　of　failure　cone　is　greater　than　that　in　NC,　increasing　from　1.5hef　to　2.5hef.　Stud　groups　in　UHPC　with　fibers　also　have　higher　pull-out　strength　and　ductility,　but　the　group　effect　is　more　significant　due　to　the　increased　overlap　of　the　failure　surface.　The　group　effect　reduces　with　the　increasing　spacing　of　studs,　but　is　not　obviously　influenced　by　the　fiber　content.　New　design　equations　are　proposed　to　estimate　the　pull-out　strength　of　single　studs　and　stud　groups　in　steel-UHPC　composite　structures.