This chapter analyses the security nexus between water, energy, food and biodiversity (WEF&B). The research question is, how could the nexus between WEF&B security be improved in a country with high environmental and social vulnerability, and which is seriously affected by climate change and organized crime? After a short conceptual review of WEF&B security, the dominant nexus is explored for Mexico, addressing first the feedbacks between water and biodiversity, and later changes in land use, food production and social vulnerability. Mexico is an oil-exporting country and has the fourth most important reserve of shale gas in the world. It has extensive drylands where 77 % of the population lives. These produce 87 % of the GDP but receive only 31 % of the water that falls as rainwater; the environment and the aquifers are thus overexploited. Furthermore, a neo-liberal free trade policy has allowed highly subsidized food imports, as well as rural–urban and international migration of peasants. Finally, extreme events influenced by climate change, such as hurricanes and droughts, have had a negative impact on human lives and on the economy. In addition, organized crime controls a part of the trade in migrants, drugs, and arms, as well as timber. A weak legal system has fostered small-scale crime, and this has increased public insecurity. As well as this, fracking activities in water-scarce regions are impacting on deep aquifers and limiting processes of adaptation to climate change in desert regions. The nexus between scarce water, overexploited aquifers, deforested areas, disasters, high food prices, weak rural government support, high energy prices and fragile governance is increasing poverty and the migration of farmers on rain-fed lands, as well as creating the risk of social instability in urban areas.