In this paper, we present CHRONOS-a tool that enables the querying, exploration, and discovery of historical change events to source code. Unlike traditional Revision-Control-System tools, CHRONOS allows queries across any subset of the code, down to the line-level, which can potentially be contiguous or disparate, even among multiple files. In addition, CHRONOS provides change history across all historical versions (i.e., it is not limited to a pairwise “diff”). The tool implements a zoom-able user interface as a visualization of the history of the queried code to provide both a high-level view of the changes, which supports pattern recognition and discovery, and a low-level view that supports semantic comprehension for tasks such as reverse engineering and identifying design rationale. In this paper, we describe use cases in which CHRONOS may be helpful, provide a motivating example to demonstrate the benefits brought by CHRONOS, and describe its visualization in detail.