Hierarchical data is very common in daily life, and tree visualization, which is used to represent hierarchical data, is an important part of visual analysis. While comparative analysis is widely used in problem solving, tree comparison plays an important role in visual analysis. From the perspective of comparing tree numbers, there are inter-tree comparison, pair-wise comparison and multi-tree comparison. Taking data characteristics into consideration, static tree comparison and dynamic tree comparison can be applied. Tree comparison can also be classified into structure comparison and attribute comparison according to comparison tasks. This paper provides an overview of the tree comparison methods relating to visualization area. These methods are divided into three categories according to their representation:juxtaposition, superposition and animation. The paper analyzes both advantages and disadvantages of different approaches, and then gives suggestions in different situations. It concludes with a discussion about the interactive explorations and the challenges of tree comparison.