过渡化石是既保留有其祖先、也保留有其演进出的后代的生命形态的生物化石。 尤其是当其演进出来的生物与其祖先完全不同时，过渡化石在科学研究上的意义就显得更加重大。这种化石暗示着人类只是用自己所认定的生物划分系统强行将一个完整的连续统给割裂开来了而已。因为化石本身的不完整性，通常无法判断一具过渡化石与进化的分歧点有多接近。因此虽然过渡化石场被当做其祖先的模型来进行研究，但并不能假设它们就是后代们的直接祖先。

当1859年达尔文出版《物种起源》时，化石记录本身亦并不为世人所熟知，更勿论过渡化石。因此达尔文曾说过渡化石的缺乏是其理论最大的弱点，但同时他也认为这可以用地质记录数据的缺乏来解释。 实际上后来其反对者也用这“缺失的一环”来攻击达尔文的学说。 然而在《物种起源》出版之后两年就出土了始祖鸟的化石，而这一化石被认为是恐龙向鸟类演进的重要过渡化石。在随后的几百年中，又有更多的过渡化石被人类发现，现在其丰富程度已经足以证实脊椎动物的纲之间都是有关系的。

A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.