QuickBooks Online API provides a robust framework for integrating financial data and processes with other applications, allowing businesses to streamline operations such as invoicing, expense management, and financial reporting. The API consists of endpoints that enable access to various QuickBooks resources, supporting both internal integrations and connections between products and customer instances of QuickBooks Online. Successful integration requires creating an OAuth app on the Intuit Developer platform, with appropriate scopes for accessing accounting or payment functionalities. Best practices for integration include using the correct API minorversions, managing data limitations in report endpoints, and employing change data capture to track deleted records. Common integration use cases involve connecting QuickBooks with CRM systems like Salesforce to automate customer invoicing, integrating with communication platforms like Slack for overdue invoice notifications, and syncing with fintech solutions to enhance user experience. Merge's Unified API offers a solution for integrating QuickBooks Online with other ERP systems, providing tools for seamless onboarding, data synchronization, and integration observability to facilitate efficient management of customer integrations.