The stages of reproductive development in groundnut (Arachis hypogaea L.) most susceptible to environmental stress were examined under a wide range of growing conditions. Experiments comprised a timing-of irrigation study in controlled-environment glasshouses in the UK, and a shading and intercropping experiment conducted during the rainy-and post-rainy seasons, respectively, in India. Peg development was relatively insensitive to environmental factors other than temperature, with the thermal time required for peg initiation differing by only 120 degree-days (°Cd) between crops. The environmental factors influencing pod yield operated mainly through their effects on the timing and duration of pod production. Water stress delayed pod initiation, and the duration of pod production was highly sensitive to both shading and water stress. The major cause of variability in pod yield and harvest index was the delay between peg initiation and onset of rapid pod growth, because once pods were initiated, the proportion of dry matter allocated to reproductive sinks was relatively conservative. Results are discussed in terms of their implications for improved breeding and management strategies.