Federated learning (FL) is a novel distributed machine learning paradigm that enables participants to collaboratively train a centralized model with privacy preservation by eliminating the requirement of data sharing. In practice, FL often involves multiple participants and requires the third party to aggregate global information to guide the update of the target participant. Therefore, many FL methods do not work well due to the training and test data of each participant may not be sampled from the same feature space and the same underlying distribution. Meanwhile, the differences in their local devices (system heterogeneity), the continuous influx of online data (incremental data), and labeled data scarcity may further influence the performance of these methods. To solve this problem, federated transfer learning (FTL), which integrates transfer learning (TL) into FL, has attracted the attention of numerous researchers. However, since FL enables a continuous share of knowledge among participants with each communication round while not allowing local data to be accessed by other participants, FTL faces many unique challenges that are not present in TL. In this survey, we focus on categorizing and reviewing the current progress on federated transfer learning, and outlining corresponding solutions and applications. Furthermore, the common setting of FTL scenarios, available datasets, and significant related research are summarized in this survey.

联邦学习（FL）是一种新颖的分布式机器学习范式，通过消除数据共享要求，使参与者能够协同训练中央模型并保护隐私。联邦学习涉及多个参与者，并要求第三方汇总全局信息以指导目标参与者的更新。为解决此问题，联邦迁移学习（FTL）吸引了众多研究者的关注。本综述聚焦于对当前联邦迁移学习进展的分类和综述，并概述相关解决方案和应用。

联邦迁移学习综述：挑战、方法与应用