To meet the basic health and disease-related needs of the world's people, particularly of those in the developing countries, exacting health system planning with emphasis on the primary level of care will be required. Regardless ofcountry or region, primary care services need to meet certain basic criteria, i.e. (i) to address particularly those health and disease conditions which are of major concern, (ii) be acceptable and compatible with the social and cultural attributes of those served, (iii) be available to the community on an on-going basis, (iv) be as effective (and safe) as the stage of scientific knowledge and financial and manpower resources will allow, (v) contribute to personal and community development. In the foreseeable future it will be impossible to extend health care coverage meeting these criteria to all persons unless usual approaches to primary health care planning and implementation are altered. Not only will attention need to be focused on the major health problems, but when conditions are selected for attention they need to be countered at a stage in progression (and at a level of service) where the smallest expenditure will have the greatest possible effect. In addition choices will need to be made among alternative technologies to ensure that the balance among factors of clinical effectiveness and safety and factors of cost acceptability, feasibility, and reliability results in actual effectiveness. Each technology proposed will, therefore, need to be examined as to this balance and whether or not modification of it will further these goals. To select among alternative strategies and their attendant technologies on this basis represents a radical departure from traditionally delivered medical care in which the decision as to which strategy or technology will be used has been left for the most part to the discretion and initiative of individual practitioners (or administrators) who have had little evidence on which to base their decisions and few resources with which to modify interventive methods and techniques. The term health technology used in this context signifies the complex of actions taken to achieve certain outcomes, and includes all the methods, techniques, instruments, equipment, drugs and biological substances making up this action. Since primary care looks to promotion of health and prevention of disease as well as the care of the sick, the range of its technology must be broad and include those of little interest to hospitals and medical centers. Its central modalities of individually directed diagnostic, therapeutic, and preventive measures and health education as well as direct actions taken to effect hazardous environmental conditions must be joined with management science, community organization, food and agriculture sciences, and many others. The question of when or where modification of a technology should occur requires a planning process which can determine for each strategy its effectiveness, safety, affordability, complexity, feasibility, reliability, and acceptability. To ensure the availability of technologies effective under the conditions of delivery in the developing countries, existing knowledge about simple, appropriate technology will need to be collected and information about it communicated for its widest possible use. Where gaps in knowledge are found, evaluative and field studies will need to be promoted and when the need for modification or adaption is found to exist, developmental projects will need to be supported. These efforts wilt require a world wide collaborative effort involving local, national and international bodies.