Bags create a closed system that reduces possibility of contamination. Swabbable needle-free valves are provided that provide repeated sterile access (and prevent needle-sticks). Our cell culture bags have high optical transmission that allows for inspection of contents without risking contamination of the culture. The bags are gas permeable and provide high rates of oxygen transfer to cells under culture. Additionally, water permeability is very low, so water loss is negligible and humidifed incubators are not required.