比较语言学，原作比较文献学，中文习称历史比较语言学。作为历史语言学的一个分支，其目标是确定语言间的历史亲缘关系。

亲缘关系即暗示着共同的来源或共同的祖语，比较语言学致力于重构（中国的语言学者习称为「构拟」）祖语并标识出已被文献记载的语言变化的痕迹。为了区别重构的形态和实际出现过的形态（指文本），一般在记录构拟词语的文本前面加上星号标记。（*）

比较语言学的基本方法是比照法，通过该方法对语音系统、形态系统、语法和语汇进行比较。原则上，在有亲缘关系的两种语言之间，各种差异都应该能够得到合乎情理的解释，也都对应着系统的演变。以比照法得出的构拟结果虽然仅是理论上的假设，但往往也会有预言的效力。比如在一个典型的实例中：索绪尔曾提出，原始印欧语的辅音系统中应包含一套喉音，但这些音在当时所知的印欧各语种中都不存在。直到后来发现了赫梯语，事实证明了索绪尔推测的正确性。

Comparative linguistics (originally comparative philology) is a branch of historical linguistics that is concerned with comparing languages to establish their historical relatedness.

Genetic relatedness implies a common origin or proto-language and comparative linguistics aims to construct language families, to reconstruct proto-languages and specify the changes that have resulted in the documented languages. To maintain a clear distinction between attested and reconstructed forms, comparative linguists prefix an asterisk to any form that is not found in surviving texts. A number of methods for carrying out language classification have been developed, ranging from simple inspection to computerised hypothesis testing. Such methods have gone through a long process of development.