In this dissertation, we explore the potential of wireless sensor networks (WSNs) in an original context, the small agriculture of Developing Countries (DCs). Our goal is to confront an emerging technology with a concrete problem of world-wide dimensions, the sustainability of farming for small land-holders living in conditions of water scarcity. Based on a survey about information needs, we design a series of precise use cases, provide system design, implementation and deployment guidelines for the technology, present a toolkit including an original interface to wireless sensors for non-specialists, and bring to the attention of the research community the lessons we learned in the process. In the first part, we present the environmental challenges faced by the developing world and identify relevant applications of environmental monitoring in this context. Then, we proceed with a review of the technology of environmental monitoring in the broad context of agriculture and formally present the opportunity represented by WSNs. Finally we show how this can be applied to addressing a crucial problem of DCs, namely rural poverty. The second part of the dissertation is devoted to the collaborative design of a decision-support tool for marginal agriculture using wireless sensor networks. We first describe a survey that was made in 2004 in three villages of Karnataka, India. The results highlighted the potential that environment-related information has for the improvement of farming strategies in the face of highly variable conditions, in particular for risk management strategies (choice of crop varieties, sowing and harvesting periods, prediction of pests and diseases, efficient use of irrigation water etc.). The results were used to identify potential use cases for an environmental monitoring system for agriculture, and to make crucial design decisions for this system. At this point, we present our toolkit in detail and proceed with its assessment. Deployment issues are covered in detail, as they are critical for the success of such a system. The results of our field deployments, both in Switzerland and in India, highlighted the potential of the technology and demonstrated its applicability in the field. However, the direct use of this technology by the farmers themselves did not foster the expected participation of the population. This made it difficult to develop the intended decision-support system. The third part of this dissertation addresses the lessons learned and their consequences for upcoming experiments and deployments. We take the following position: Currently, the deployment of WSN technology in developing regions is more likely to be effective if it targets scientists and technical personnel as users, rather than the farmers themselves. We base this finding on the lessons learned from the COMMON-Sense system deployment and the results of an extensive user experiment with agriculture scientists, which is extensively described. We also took steps to make the deployment and maintenance of wireless sensors easier. Their limited resources, indeed, make them a challenging tool to handle in the field. In particular, they lack a proper display, which makes them difficult to deploy and to manage, once they are deployed. Accordingly, we present Sensor-Tune, a light-weight deployment and maintenance support tool for wireless sensor networks. This tool is based on an auditory user interface using sonification. Sonification refers to the use of audio signals (mostly non-speech) to convey information. We explore the potential of this approach, in particular how it allows users to overcome the inherent limitations of visual interfaces. We then justify our design choices, and present typical WSN applications where sonification can be particularly useful. Finally, we present the prototype that we built, and we describe a user experiment that we conducted in early 2008, which is the first reported attempt to put a multi-hop wireless sensor network deployment in the hands of non-specialists. In a conclusive part, we go beyond the mere technology and technology use, by advocating an original use of Information and Communication Technologies (ICT) in the context of developing countries. We believe our demand-driven approach for the design of appropriate ICT tools that are targeted at the resource poor to be relatively new. In order to go beyond a pure technocratic approach, we adopted an iterative, participatory methodology. This work represents the first reported attempt to design and deploy a wireless sensor network in the rural area of a developing country. In this context, the author hopes that the lessons learned and the results obtained in the field, with both their limitations and their promises, can inspire future work on environmental sustainability in developing regions.