The polymer electrolyte membrane fuel cells (PEMFCs) are electrochemical devices that convert the chemical energy of hydrogen (fuel) and oxygen (oxidant) into electrical energy. Their main distinguishing features make them attractive for portable, stationary and transportation power generation. The possibility of having stationary fuel cells at a specific location may result in a decentralization of power grids, reduction of transmission and distribution losses. These are some of the reasons why they are considered promising to obtain an efficient and clean power generation in the twenty-first century. One of the most important components of this technology is the proton exchange membrane. This has oriented research to the improvement of its performance, durability and cost. The focus was on the ones with high proton conductivity; low electroosmotic drag coefficient, electronic conductivity and permeability to fuel; good mechanical properties and chemical/thermal stability. At present, this technology is based on perflourinated proton exchange membranes that operate efficiently only under fully hydrated conditions. When a membrane is evaluated for potential use in fuel cells, the first characteristic taken into account is its proton conductivity. This feature often depends on its water content. High proton conductivity is supported by elevated level of water uptake and high conductivity is required at high current density. This manuscript describes the widely accepted mechanisms of proton transport in hydrated polymeric membranes: Grotthus (proton hopping) and vehicular. Then it makes a review on different techniques to hydrate Nafion membranes, the steps involved in the processes, the chemical reagents used, the operating conditions and the results obtained, from the studies carried out in each of the cases.