Bagrada hilaris (Burmeister) (Hemiptera: Heteroptera: Pentatomidae) is an Old World pest recently established in North America. Literature on the life history and immature descriptions of B. hilaris is scattered and variable in content. Therefore, we conducted a study of this insect's life history in southern New Mexico from January 2012 to August 2014, reared the bugs in the laboratory, and described the immature stages. Two primary host plants, London rocket (Sisymbrium irio L., a winter annual) and mesa pepperwort (Lepidium alyssoides A. Gray, a summer perennial), were sampled weekly to record numbers of the various life stages and behavioral activities; broccoli (Brassica oleracea L.) also was sampled for one season. Adults were found continuously throughout the year. They deposited eggs individually in the soil near the bases of their host plants from February through October. Nymphs were found every month of the year. The number of generations per year was difficult to determine due to such factors as extreme overlap of generations and a lack of reproductive diapause. However, the data roughly suggest this species is bivoltine, although there is some evidence of a partial third generation. The bug also was reared from egg to adult under controlled laboratory conditions on fruiting structures of mesa pepperwort at 25 ± 0.01°C under a photoperiod of 14:10 (L:D) h. The incubation period averaged 7.45 d. The five stadia averaged 3.35, 7.08, 6.39, 7.33, and 10.25 d, respectively. Instars can be distinguished readily by differences in several morphological features in addition to body size and coloration.