As software systems are growing in complexity and size, reliability becomes a major concern. A large degree of industrial and academic efforts for increasing software reliability are directed towards design, testing and validation—activities performed before the software is deployed. While such activities are fundamental for achieving high levels of confidence in software systems, bugs still occur after deployment resulting in costly software failures. This dissertation presents assertion-based repair, a novel approach for error recovery from insidious bugs that occur after the system is deployed. It describes the design and implementation of a repair framework for Java programs and evaluates the efficiency and effectiveness of the approach on repairing data structure errors in both software libraries and open-source stand-alone applications. Our approach introduces a new form of assertions, assertAndRepair, for developers to use when checking the consistency of the data structures manipulated by their programs with respect to a set of desired structural and data properties. The developer provides the properties in a Java boolean method, repOk, which returns a truth value based on whether a given data structure satisfies these properties. Upon an assertion violation due to a faulty structure, instead of terminating the execution, the structure is repaired, i.e., its fields are mutated such that the resulting structure satisfies the desired properties, and the program proceeds with its execution. To aid developers in detecting the causes of the fault, repair-logs are generated which provide useful information about the performed mutations. The repair process is performed using a novel algorithm that uses a systematic search based on symbolic execution to determine valuations for the structures’ fields that result in a valid structure. Our experiments on repairing both library data structures, as well as, stand-alone applications demonstrate the utility and efficiency of the approach in repairing large structures, enabling programs to recover from crippling errors and proceed with their executions. Assertion-based repair presents a novel post-deployment mechanism that integrates with existing and newly developed software, providing them with the defensive ability to recover from unexpected runtime errors. Programmers already understand the advantages of using assertions and are comfortable with writing them. Providing new analyses and powerful extensions for them presents an attractive direction towards building more reliable software. / text