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Solution annealed (SA) and 20% cold worked (CW) Type 316 (UNS S 31600) stainless steels were irradiated in the high flux isotope reactor (HFIR) to 55 dpa and to 3445 appm helium at 300 to 500°C. The irradiated specimens were examined by transmission electron microscopy (TEM) to evaluate microstructural evolution. At irradiation temperatures of 400°C and below, there was very little difference in the microstructural and swelling behavior between SA and CW material. A high concentration of fine bubbles was uniformly dispersed in the matrix. Swelling was very low (<0.4%). High concentrations of dislocation loops were observed. No precipitates were found at 300°C. A few precipitates were observed in the matrix and along the grain boundaries at 400°C. Large differences in swelling behavior and in the formation of precipitates appeared between 400 and 500°C. Low concentrations of large dislocation loops were observed in both materials at 500°C. The cavity size distributions were broad, but clearly bimodal. Swelling values in SA and CW were 2.3 and 0.7%, respectively. Many coarse precipitates were found in the matrix of SA, which were identified as M6C type phase enriched in nickel, iron, chromium, silicon, and molybdenum. The CW material developed fewer precipitates than the SA material at 500°C. Swelling behavior was compared in heat-to-heat variations of Type 316 stainless steel irradiated in HFIR. Significant swelling differences between heats are seen for irradiations in the temperature range of 500°C and above and at fluences beyond 30 dpa.