Virtualization has become the norm over the past decade, but multi-tenancy is not the norm in typical on-premises environments. Instead, it is associated with cloud computing. Both are critical because they drive cost advantages and enable the provider to offer a more competitive subscription rate. We have experience hosting single-tenant systems, under monikers like strategic outsourcing, managed operations, and managed service delivery. There is just no comparison. Most customers come to the cloud to save IT cost, both in terms of avoiding the larger up front license cost, in term of paying only for what you use in the case of so-called metered services, and in terms of a lower overall total cost. And those savings are driven by multi-tenancy and virtualization. Period. Not all systems are inherently architected to be multi-tenant systems, but the overall cloud solution must be. The Blackberry Enterprise Server (BES) is an example in our LotusLive (SmartCloud) environment. BES does not currently have a multi-tenant architecture. To offer a cost competitive BES service to our customers wanting to receive mail on their Blackberries, we've implemented a multi-tenancy architecture on our side connecting into BES without changes needed to the BES source code. If cost is the primary objective, there is no substitute for multi-tenancy; it is essential to cost reduction. Needless to say, both architecture, design, coding and testing have to emphasize prevention of cross-over visibility between tenants. Since multi-tenancy is basically new and rarely implemented in on-premises solutions, there are entire suites of test cases to be added for cloud solutions to verify the complete separation of tenants. Both design and test need to carefully plan around the multi-tenant architecture.