A key assignment scheme is a cryptographic technique for implementing an information flow policy, sometimes known as hierarchical access control. All the research to date on key assignment schemes has focused on particular encryption techniques rather than an analysis of what features are required of such a scheme. To remedy this we propose a family of generic key assignment schemes and compare their respective advantages. We note that every scheme in the literature is simply an instance of one of our generic schemes. We then conduct an analysis of the Akl-Taylor scheme and propose a number of improvements. We also demonstrate that many of the criticisms that have been made of this scheme in respect of key udpates are unfounded. Finally, exploiting the deeper understanding we have acquired of key assignment schemes, we introduce a technique for exploiting the respective advantages of different schemes.