Policy as Code is a method for managing infrastructure operations by applying software engineering practices like automated testing, deployment, and version control to policy creation. Pulumi's CrossGuard solution facilitates this by enabling the creation, verification, application, and enforcement of policies across various cloud providers such as AWS, Azure, Google Cloud, and Kubernetes. These policies are language-agnostic and can be written in TypeScript, JavaScript, or Python, allowing for versatile application. CrossGuard's key features include a Policy SDK for custom policy coding, local infrastructure validation before deployment, and Policy Packs that enforce best practices for security, reliability, and cost management. Policies, which comprise a name, description, enforcement level, resource validation, and violation message, are categorized into ResourceValidationPolicy and StackValidationPolicy, ensuring the adherence to specific logic and validation of resources within a stack. Examples provided demonstrate how policies can prevent public access to resources across multiple cloud platforms, showcasing the comprehensive capabilities of Policy as Code in enforcing infrastructure compliance and security.