The "hard gainer" of the fitness world since they have a hard time building muscle although it is not impossible.
They're naturally small built with long, lightly muscular limbs. The have a fast metabolism so the can stuff their faces without putting on weight.
To gain muscle they need to eat alot and do heavy weight training, its advised that they avoid cardiovascular activities since cardiovascular activities burn fat
Examples of ectomorphs are Lisa Kudrow, Kate Moss, Brad Pitt, Seth Green, Edward Norton and most NBA players