Indigenous and local peoples are increasingly being recognised to play an important role in the global environmental science-policy arena. In this context, participatory environmental monitoring (PEM) is promoted as a cost-effective approach to collect and report data on environmental trends and support decision-making at different scales while providing significant social co-benefits to local people. However, in practice PEM refers to a plethora of approaches and there is little common understanding about what is meant by ‘participatory’, i.e. to what extent are local peoples’ interests, motivations and perceptions taken into account in designing the schemes. In addition, cases that involve local peoples’ ecological knowledge in PEM rarely investigate local knowledge systems or assess the validity and consistency of such knowledge. This PhD dissertation is the result of a three-year research aiming to explore the role of local and indigenous peoples’ participation in environmental monitoring, with special focus on the social dimension of participatory processes, local ecological knowledge and monitoring of biodiversity and deforestation. The dissertation specifically investigates 1) the meaning of ‘local participation’ in PEM schemes; 2) the motivations of the local people who engage in PEM; 3) aspects of local ecological knowledge (LEK) important in PEM, including determinants of plant identification skills and complementarities between folk and scientific knowledge systems; and 4) local peoples’ perceptions of participation in PEM. To analyse local participation in environmental monitoring and motivations, the research involved a review of case studies across the world (Paper I) and an empirical case study of an autonomous local monitoring group in Cambodia (Paper II). The study of aspects of LEK important in PEM was addressed through three case studies in Southeast Asia: Indonesia, Vietnam (Paper III) and Cambodia (Paper IV). Perceptions of local people towards PEM were gathered for all case studies. The research involved approximately nine months of fieldwork and a wide range of methods were used, including focus group discussions, structured interviews, plot based forest inventories, expert- and participatory plant collections, field identifications and herbaria work. The results of the literature review (Paper I) show that the role of local people in PEM was generally reduced to data collection, and that they were rarely involved in deciding what to monitor, how, and what the data can be used for. Results showed that projects most often x prioritised ‘environmental monitoring’ over ‘participation’ limiting the co-benefits and empowerment potential of PEM. Autonomous local monitoring was driven by livelihood concerns and the motivation to protect the forest from outsiders (Paper II). Differences in motivations and interests between local peoples and researchers (Papers I & II) may lead to a decrease of local participation in PEM over time or, at least, locally-driven monitoring may eventually become detached from non-local wishes and objectives. Future studies and projects should better seek to align the scientific/professional aims with local objectives and needs, for both parties to benefit from the project and provide opportunities for local empowerment. The studies show that in order to successfully involve LEK in PEM with scientific purposes, it is important to assess: i) participants’ sociodemographic and economic profiles (Papers II & III); ii) access to and use of the natural resources –or other attributes– being monitored (Papers II & III), and in the case of monitoring plant resources iii) usefulness of the species, iv) significance of the individual to be identified (e.g. landmarks, sacred trees) and v) “ease” of identification (Paper III). This dissertation contributes with theoretical and methodological knowledge on how to involve local peoples and their knowledge in meaningful ways in PEM, which can potentially be used to implement global initiatives for forest and biodiversity protection (e.g. CBD, IPBES, SDG and REDD+).