Scalability is an important issue in the construction of distributed systems. A number of theoretical and experimental studies have been made on scalability of distributed systems. However, they have been either studies on specific technologies or have studied scalability in isolation. The main conjecture of our work is that scalability must be perceived along with the related issues of availability, synchronization and consistency. In this context, we propose a scalability model which characterizes scalability as being dependent on these factors as well as the workload and faultload. The model is generic and can be used to compare scalability of similar systems. We illustrate this by a comparison between NFS and AFS, two well known distributed file systems. The model is also useful in identifying scalability bottlenecks in distributed systems. We have applied the model to optimize Virat, a wide-area shared object space that we have built.