Direct methanol fuel cells (DMFCs), employing liquid methanol as a fuel, offer an attractive option in portable devices due to their simplicity in the system structure (easy storage and supply), and no need for fuel reforming or humidification. For obtaining a higher power density, the membranes that show high proton conductivity, and at the same time, low methanol permeability are strongly desired. However, there is only a little progress because of trade-off relations between these parameters. Also the membrane stability, particular to hydrolytic and chemical stability is recognized as a key factor that affects fuel cell performances. In the authors' recent work, they have been working on the design and the development of new families of cost-effective, readily prepared proton-conducting membranes based on chemically cross-linked PVA-PAMPS [poly(vinyl alcohol) and poly(2-acrylamido-2-methyl-1-propanesulfonic acid)] composites. The authors have first introduced new concepts of secondary polymer chains such as 'binary chemical cross-linking' or 'hydrophobiciser' and the 'stabiliser' effect. Also, the authors have established a new concept of PVA-PAMPS based semi-interpenetrating polymer networks (semi-IPNs) by incorporating plasticizer variants R (R = poly(ethylene glycol)(PEG), poly(ethylene glycol) methyl ether (PEGME), poly(ethylene glycol) dimethyl ether (PEGDE), poly(ethylene glycol) diglycidyl ether (PEGDCE)) and poly(ethylene glycol)bis(carboxymethyl)ether (PEGBCME) as the third components. Incorporation of the above concepts promoted not only the high proton conductivity, flexibility with low methanol permeability (1/3 - 1/2 of Nafion 117 membrane), but also the excellent hydrolytic and the oxidative stability of PVA-PAMPS composites. The membrane electrode assembly (MEA) fabricated with PVA-PAMPS composites has been successfully established, which showed the similar open circuit voltage (OCV) to that of Nafion 115, and a power density 52 mW cm-2 at 80 degrees Celsius. A striking feature of the long-term test was that no appreciable decay of the current density was observed during the whole operation time longer than 130 hours at 50 degrees Celsius, and so was the power density. This book is the first time that such long-term operation of DMFC was reported since PVA-PAMPS composite are all hydrocarbon membranes made simply of aliphatic skeletons. They are very different from the perfluorosulfonic membranes such as Nafion, or other reported membranes with aromatic skeletons. Therefore this affords the PVA-PAMPS composites unique structure compared to most of the proposed membranes, which suggests the good candidacy of PVA-PAMPS composites when they are intended for use in low temperature DMFCs.