  We analyze pairing in two dimensional spin liquids. We argue that interplane
pairing enhanced by magnetic correlations is the most plausible explanation of
the spin gap phenomenon observed in underdoped cuprates. The details of the
pairing theory depend on the in-plane antiferromagnetic correlations. We
consider two models: $2p_F$ correlations induced by a strong gauge field
interaction and undamped spin waves. We estimate the pairing temperature $T_s$
and the angular dependence of the gap function and discuss physical
consequences.
