The present disclosure relates to semiconductor structures and, more particularly, to asymmetric source and drain structures and methods of manufacture. The structure includes: at least one gate structure; a straight spacer adjacent to the at least one gate structure; and an L-shaped spacer on a side of the at least one gate structure opposing the straight spacer, the L-shaped spacer extending a first diffusion region further away from the at least one gate structure than the straight spacer extends a second diffusion region on a second side away from the at least one gate structure.