<div>Execution comparison techniques compare multiple executions from the same program or highly similar programs to identify state differences including control flow differences and variable value differences. Execution comparison has been used to debug sequential, concurrent, and regression failures, by reasoning about the causality between execution differences and input differences, thread scheduling differences, and syntactic differences among program versions, respectively. However, execution comparison techniques have several limitations. First, executions may have benign differences, which are not related to the behavior differences that the user can observe. Second, huge storage spaces are required to record two independent executions. Third, the techniques can only compare executions from the same or similar programs.</div><div><br></div><div>In this dissertation, we present an execution comparison technique that (1) removes benign differences, (2) requires less space, and (3) can compare two different programs implementing similar algorithms. Also, we present that the execution comparison technique can be used in identifying and extracting a functional component out of a binary. First, we present a dual execution engine that executes multiple executions at the same time and only introduces the desired differences. Also, the engine compares the executions on-the-fly and stores the differences only. Second, we present a technique to compare two programs written by two different programmers. Especially we will show that this technique can compare the buggy program from a student and the correct from the instructor and can reason about the errors.</div><div><br></div>