Modern SaaS architectures increasingly require advanced entitlement management systems that integrate with usage-based billing models, a necessity given the shift toward metered billing, with 38% of SaaS companies already adopting this model and 74% expected to do so by the end of 2023. Entitlements in SaaS refer to granular permissions and access controls that define customer access to features, resources, and service levels based on their subscription and usage patterns, necessitating sophisticated systems that can handle real-time usage tracking and dynamic pricing. Implementing metered billing can enhance net revenue retention, as demonstrated by HubSpot's increase from 70% to nearly 100% after adopting this model. Key architectural elements include event-driven processing for real-time entitlement and usage tracking, integration with billing platforms for accurate invoice generation, and operational best practices like real-time usage dashboards and automated customer notifications. As SaaS businesses scale, managing entitlement complexity becomes crucial, and solutions like Lago offer integrated platforms to handle high-volume events with sophisticated entitlement enforcement, supporting various pricing models while ensuring operational efficiency and customer transparency.