The protein kinase D (PKD) enzymes represent a new family of second messenger stimulated kinases, with diacylglycerol as a prime, but not the sole, mediator of activation. Their molecular architecture features a catalytic domain, unrelated to that of all PKC family members, and a large inhibitory, regulatory domain, comprised of two Zinc fingers, and a pleckstrin homology domain. These different sub-domains play distinctive roles in the activation, translocation and biological functions of the kinase. The enzymes have been implicated in signalling mechanisms controlling cell proliferation and programmed cell death and in metastasis, immune responses, and Golgi restructuring and function. A variety of proteins specifically interact with the different sub-domains of the enzymes and direct their wide range of cellular functions.