Many small libraries, such as those endowed in Parish Churches in the UK, are subject to climatic conditions that are outside the limits commonly accepted as correct ( such as those referred to in British Standard BS 5454). However, inspections of many such libraries suggest that they suffer little harm if the books are cold and well ventilated. Mould attack on books can be divided into various classes. 1. Local - due to presence of hygroscopic materials, or accidental wetting outside the room. 2. General - due to excessive relative humitity. 3. Foxing - in high relative humidity. 4. External - due to wetting from leaks or invasive dry rot. The characteristics of each class are described, and used as a guide to the sensitivity of attack, the nature of attack and the causes and remedies. The reasons for the adoption of the present climatic limits are given and the importance of flexibility of interpretation stressed.