The use of infrared spectroscopy (IR) in agriculture and environmental services represents a major advance in technology and field diagnostics. The technique uses only light for rapid, non-destructive analysis of soil and plant materials and is similar in many respects to digital photography: reflectance from a sample of material is collected across a range of infrared wavelengths beyond those that can be detected by the human eye. Working from a digital scan, a “reflectance fingerprint” is obtained from which technicians can easily predict the nature of multiple soil and plant properties. The technique is extremely economical and does not require the purchase of costly chemicals or other consumables normally used in conventional soil and plant analysis. Tropical developing countries stand to gain most from the rapid adoption of IR technology towards achievement of the Millennium Development Goals on poverty and the environment. Examples are given on how IR technology is being used to implement an evidence-based approach (clinical agronomy) to land degradation assessment and agricultural advisory services in developing countries. Remote sensing and GIS information is used to lay down spatially stratified sampling schemes for field sample collection. IR is used to characterize soil, sediment, plant tissue, organic resource, and livestock faecal samples from geo-referenced locations. IR-predicted information on soil, crop and livestock health constraints is spatially interpolated and mapped by calibration to remote sensing imagery. The IR-based assessment framework provides a basis for (1) rapid quantitative diagnosis of constraints to soil, plant and livestock health in a target area, including quantification of environmental and socio-economic risk factors; (2) development of site indices base on calibration off crop and tree growth measured in field response to IR soil and plant tissue spectra; and (3) baselines and monitoring schemes for scientifically-rigorous impact assessment of development interventions on soil, plant and livestock health. IR technology can help to bring soil and plant testing services to resource-poor smallholder farmers in developing countries. Rapid acquisition of information on soil organic carbon and soil functional capacity using IR can enable monitoring and assessment of compliance in environmental service payment schemes at low cost. IR technology is providing a basis for whole new soil interpretation systems that largely by-pass conventional soil and plant testing. The same technology has many additional applications in quality analysis of agricultural inputs and products, so that developing country laboratories can use one instrument for multiple purposes. Issues concerning setting up IR laboratory networks in developing countries are discussed, including instrument stability and calibration transfer. Guidelines are given on instrument options, including dispersive versus Fourier-transform spectrometers and differences between near-infrared, mid-infrared, and attenuated total reflectance spectroscopy.