In most studies, hypertension in children has been secondary to an identifiable cause in a large majority of those studied (1,2). This has changed during the relatively recent epidemic of childhood obesity, where primary hypertension in many centers now is the most common cause form of hypertension (3). In adults primary hypertension is the dominating diagnosis. This chapter will discuss those cases where a cause for the hypertension has been possible to identify. It is quite possible that in the future some children that presently are diagnosed with primary or essential hypertension will be found to have an underlying diagnosis.