Business Process Rules specify the manner in which the business process of an application would function. They form a part of the core business logic of the applications and are important, if not critical, for an enterprise to facilitate agility in their business processes; enabling them to be adaptive for future changes. Applications today should be flexible and adaptive to business process changes. This can only be ensured by adhering to a declarative approach in the modeling of business processes. Business Rules are now being used widely for their capability to make the business processes agile and transparent. The isolation of the Business Rules from the application’s code using Business Rules Engine is the key to greater flexibility and better adaptability for future changes in the Business Process Management Systems (BPMS). This article has discussed Business Process Rules and its applicability in designing flexible, agile systems.