This thesis is grounded in the introduction of participatory research, development and extension (RDE) to Australian agriculture. The emphasis on participatory processes emerged as the Transfer-of-Technology (ToT) model is no longer considered adequate to deal with complex farming systems and their diverse stakeholders (Packham 2003). However, RDE agencies are introducing participatory processes with a limited understanding of how they may work in Australia's developed agriculture sector (Vanclay 1994). Consequently, the initiation of three participatory Farming Systems RDE projects in Australia's northern grains region provides opportunity to explore and understand participatory approaches and their impact on participants. Three related themes are developed by exploring the nature of participants' diversity across these projects, the characteristics of participation in each project and the subsequent learning of participants: (i) that increased participation implicitly increases diversity in the conduct of RDE; (ii) that participatory RDE provides opportunities to integrate stakeholders' diverse experience and knowledge; and (iii) that participants' learning can improve current farming systems. Participatory action research was used to understand how participatory RDE should be enacted, and to involve project participants to help them also understand and improve their project processes and goals. The research utilised a range of qualitative and quantitative procedures including: participant and nonparticipant observation at project meetings and activities with farmers; focus groups and semi-structured interviews with project teams, their managers, and participating farmers; a team learning survey of team members; and custom-made questionnaires to quantify participants' perceptions of the projects, their processes, and impacts on learning and behaviour. These interventions identified participants' demographic, organisational and informational diversity. They also identified and elaborated their diverse aims, expectations and passions for participatory RDE, revealed individuals' preferred RDE methodologies and suggested their underlying worldviews. Indeed, ToT paradigms and positivist worldviews remained entrenched in most project staff and their managers. The teams consequently used participation to help farmers better understand technical issues, not build interdependent projects with integrated RDE processes. Farmers generally appreciated their increased participation and influence in RDE activities. While their initial consultative and functional participation did not extend to process decisions or project strategy, farmers valued the opportunity to work with the projects and influence the issues addressed. The projects consequently improved farming practices and management of issues that have long-eluded traditional RDE. For example, the widespread use of: (i) zero tillage and controlled traffic systems to control soil erosion; (ii) nitrogen fertilisers to match crop requirements; (iii) ley pastures to address soil fertility in grain and grazing systems; and (iv) new crops to diversifying grain systems away from monocultures. The projects provided farmers with opportunities for increased participation and learning over time. Functional participation developed in on-farm research that addressed issues identified with farmers, and action learning workshops provided proximal opportunities for farmers to understand existing information and use their own farm data in real decisions. In this way, the behaviourist learning of the ToT approach was supplemented by guided learning to integrate new meaning schemes with farmers' experiential knowledge. Some on-farm research and action learning activities extended to reflecting on their processes, and involved farmers in deciding the most appropriate RDE methodology and methods for subsequent activities. This opportunity to reflect on the values and assumptions of different approaches was critical in developing interactive participation and higher level learning for participants. Nevertheless, the initial participation in each project team failed to meet some team members' expectations. The expected task and process conflicts emerged, but small activity groups with shared values and RDE paradigms developed within each team. Team members' process conflict about the 'best' RDE methodologies for specific issues then developed into worldview conflicts about the relevance and rigour of these methodologies. Some smaller groups subsequently worked independently, with damaging relationship conflict developing from unresolved process issues between some individuals. Team members communicated, but their participation remained largely passive and consultative. Factors that shaped participation were identified, and a framework to support opportunities for stakeholders to plan, manage and evaluate RDE was developed. These helped increase participation in the projects. Participation within the projects' constituent activities subsequently fluxed from isolation to interactive participation. Individuals within activities now expected, and usually had, equality in content decisions (i.e. functional participation), which often extended to process decisions (i.e. interactive participation). Yet, the levels of participation between members of different activities varied across the projects. One project remained a series of parallel and relatively independent activities with passive and consultative participation. A second project had functional and interactive participation imposed for some activities, but otherwise used passive and consultative participation. The third project developed to provide functional and interactive participation in major project decisions. It then became apparent that the learning outcomes of each project varied. Individuals continued to learn from their participation in the projects, but the contributions of diverse sources and participation were major shapers of this learning. Participation within the less diverse activity groups produced mainly technical learning through participants' existing meaning schemes. Again, the level of participation between activities with diverse values, RDE paradigms, and worldviews, shaped the nature of learning. Passive and consultative participation produced mostly technical learning through existing meaning schemes. Yet, teams that embraced their diversity, and reflected on the assumptions of their different RDE methodologies, transformed their approach to learning. With high levels of diversity, the level of participation determined the level of learning. Essentially, participation became learning. This thesis confirms the potential of participatory RDE to improve farming practices. Consultation to identify priority issues, and functional participation to develop proximal opportunities for farmers to understand these issues and make their own decisions had a major impact on farming practices. Yet, participatory processes must rise above the prevailing ToT paradigms of RDE agencies to integrate participants' knowledge, and so achieve sustainable development in Australia. Three main process contributions are made to support this development. Firstly, the evaluation framework provides a challenge and structure to encourage the contributions of all participants at each stage of project activities. It provides a checklist for effective participation in Farming Systems RDE. Secondly, a typology of participation in Farming Systems RDE extends that proposed by Pretty (1995). It provides a catalyst and means to better understand and identify the most appropriate levels of participation in RDE projects. The associated checklist for assessing modes of participation allows monitoring of the participation developed in practice. Finally, the re-conceptualisation of a broader continuum of participation in Farming Systems RDE for developed agriculture is proposed. The subsequent development of the Doing successful on-farm research process is a culmination of the understandings developed in this thesis. It facilitates the development of interactive participation within the on-farm research process that is central to these projects. Recognising the technical focus of most current RDE agency staff, it guides their development of participatory on-farm research processes before reflecting on the appropriateness of different research methods to their research issues. The findings here cannot ensure the development of Australia's RDE beyond the ToT paradigm. However, this thesis provides important insights into the nature of diversity, participation, and learning in the Farming Systems projects, and a series of tools to support this development. Conceptually, it proposes that different kinds of participation will be shaped by participants' diversity from their prior experiences and their expectations that are in turn transformed through evidence of improved practice.