Very large, software-intensive systems are normally built from a combination of established legacy components and new components contributing new functionality. The cost of building such systems and maintaining them through-life can be reduced by adopting the principles and architectures of Open Systems. These principles and architectures will be discussed. The case for the benefits to be obtained from Open Systems will be argued.

Examples of Open Systems will be drawn from defense applications, from open source applications and from web services, in support of this thesis. It is hoped that the presentation will engender a lively debate, which will allow the speaker to tailor the content to the types of application of interest to the audience.