Egg and nymphal development were studied under constant temperatures for the newly introduced pest species, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae). Development was assessed at seven constant temperatures (15–35°C). Development to adult was completed at temperatures between 17 and 33°C, with egg hatch also occurring at 15°C. The relationship between temperature and developmental rate was evaluated using three developmental models. Of the models evaluated, the Briere-1 model was the best fit for the empirical data of egg and total development and for providing accurate values for the temperature threshold. Application of the linear degree-day model estimated 537.63 DD are needed for total development (egg to imaginal ecdysis). An additional 147.65 DD are needed for the preoviposition period of the female. Reproductive parameters were evaluated at 25°C and indicate a median number of 28 eggs per egg mass. Oviposition occurred at 4.32-d intervals, and a female can continue to oviposit throughout its lifespan. H. halys is univoltine in New Jersey and Pennsylvania, but if it spreads to warmer climates in the United States, it could have multiple generations per year.