A new analytical approach to water and gas coning has been developed, based on the hypothesis of gravity drainage and transient flow. The equations have been derived for infinite and confined reservoir, and they apply both to vertical and horizontal wells. Besides the usual parameters, i.e. critical oil rate and breakthrough time, this methodology allows the determination of the water cut for a variable oil rate (history match), and the determination of the maximum oil rate, variable with time, for an imposed water cut (production forecast). The method has been extensively tested using published data and a numerical simulator, with satisfactory results.