Conditions suitable for microbial activity on textiles are often described in parameters that do not correspond to the terminology, data, or theories of moisture absorption and desorption of textile fibers and fabrics. Current and historic wet processes of textile technology are profoundly affected by the chemical and physical properties of the textiles, including their potential as nutrient sources for microbial growth. Especially for hygroscopic fibers, a continuing confusion about the nature of hysteresis exists. The equilibrium moisture regains and moisture contents occur at vastly different rates. Mathematical formulae used to consider the multi-layer absorption and desorption are complex. Commercial regain rates were established to regulate the practical and economic ramifications of the inherent tendency of fibers to absorb moisture. Studies of the propensity of these fibers to support microbial growth show a much higher threshold of Relative Humidity than articulated in museum literature; textile testing laboratory conditions also (65% RH) contradict museum rules. Complex changes in strength and dimension – swelling, hygral expansion, and shrinkage, also accompany changes in the moisture regain of fibers, yarns, and fabrics. Microbial growth can and does occur on textiles, but the actual condition for growth within museums and private collections may be less straightforward and less well defined than assumed.