Phospholipase D (PLD) is a family of diverse enzymes that are differentially regulated by Ca(2+), polyphosphoinositides, free fatty acids, G-proteins, N-acylethanolamines, and membrane lipid environments. Two new types of PLDs were identified in the past year: one is activated by oleic acid and the other requires no cation for activity. The oleate-stimulated PLD is associated with the plasma membrane and binds to microtubules. The Ca(2+)-independent PLD contains a PX and a PH domain, but not the Ca(2+)/phospholipid-binding C2 domain found in most plant PLDs. The mechanism by which Ca(2+), phosphoinositides, and G proteins regulate certain PLDs is better understood. PLDs and their product phosphatidic acid are involved in various stress responses, including water deficits, salts, wounding, and elicitation. Increasing evidence supports a role of PLD in the abscisic acid signaling cascades.