The life history of the assassin bug Sinea diadema (F.) was studied in southern Illinois from February 2001 to November 2002, and the immature stages were described. The bug also was reared under controlled laboratory conditions. This bivoltine species occurs in herbaceous fields, often in association with leaves and stems of Solidago missouriensis Nuttall, and preys primarily on small bugs and beetles. It apparently overwinters as eggs. Nymphs emerged in mid-April and were found until mid-September. Adults were found from the third week of May until early October. The bug was reared in the laboratory on larval beet armyworm, Spodoptera exigua (Hübner), at 26 ± 3.0°C under a 16:8 (L:D) h photoperiod. The incubation period averaged 13.87 d; eyespots appeared in ≈7 d. The five stadia averaged 9.59, 7.80, 8.95, 11.80, and 12.97 d, respectively. Instars can be distinguished by differences in several anatomical features, including body length and width and progressive development of size, number, and pattern of spines.