One of the most common reactions of diazo compounds with alkenes is cyclopropanation, which occurs through metal carbene or free carbene intermediates. Alternative functionalization of alkenes with diazo compounds is limited, and a methodology for the addition of the elements of Z-CHR2 (with Z = H or heteroatom, and CHR2 originates from N-2=CR2) across a carbon-carbon double bond has not been reported. Here we report a novel reaction of diazo compounds utilizing a radical-mediated addition strategy to achieve difunctionalization of diverse alkenes. Diazo compounds are transformed to carbon radicals with a photocatalyst or an iron catalyst through PCET processes. The carbon radical selectively adds to diverse alkenes, delivering new carbon radical species, and then forms products through hydroalkylation by thiol-assisted hydrogen atom transfer (HAT), or forms azidoalkylation products through an iron catalytic cycle. These two processes are highly complementary, proceed under mild reaction conditions, and show high functional group tolerance. Furthermore, both transformations are successfully performed on a gram-scale, and diverse gamma-amino esters, gamma-amino alcohols, and complex spirolactams are easily prepared with commercially available reagents. Mechanistic studies reveal the plausible pathways that link the two processes and explain the unique advantages of each.