Abstract. Standardization of infrastructure and services in distributed applications and frameworks requires ground methodological base. Design by Contract approach looks very promising as a candidate. It helps to obtain component-wise design, to separate concerns between developers accurately, and makes development of high quality complex systems a manageable process. Unfortunately, in its classic form it can hardly be applied to distributed network applications because of lack of adequate means to describe nondeterministic asynchronous events. We extend Design by Contract with capabilities to describe callbacks and asynchronous communication between components. The resulting method was used to specify distributed applications and to develop conformance test suites for them in automated manner. Specifications are developed in an extension of C language that makes them clear and useful for industrial developers and decreases greatly test construction effort. Practical results of numerous successful applications of the method are described. More information on the applications of the method can be found at the site of RedVerst group of ISP RAS [1].