Cloud-based systems are used to deliver business-critical and safety-critical services in domains ranging from e-commerce and e-government to finance and healthcare. Many of these systems must comply with strict non-functional requirements while evolving in order to adapt to changing workloads and environments. To achieve this compliance, formal techniques traditionally employed to verify the non-functional properties of critical systems at design time must also be used during their operation. We describe how a formal technique called runtime quantitative verification can be used to verify cloud-based systems continually.