These chairs are Le Bembole chairs by B&B Italia. B&B is an incredible furniture manufacturer, but I especially love their sofas and armchairs. This particular style also comes in a two and three seater.
The beautiful thing about this chair is its natural organic shape, and the apparent absence of a frame.
This is a copy of a chair they did in the seventies, we own a similar type of sofa and armchair, only ours are the predecessor to ths as the frame is more apparent. (via B&B Italia)