Title: Reducing Forgetting In Federated Learning with Truncated Cross-Entropy. Abstract: In Federated Learning, a global model is learned by aggregating model updates computed from a set of client nodes, each having their own data. A key challenge in federated learning is the heterogeneity of data across clients whose data distributions differ from one another. Standard federated learning algorithms perform multiple gradient steps before synchronizing the model, which can lead to clients overly minimizing their local objective and diverging from other client solutions, particularly in the supervised learning setting. We demonstrate that in such a setting, individual client models experience the ``catastrophic forgetting" phenomenon with respect to other client data. We propose a simple yet efficient approach that  modifies the cross-entropy objective on a per-client basis such that classes outside a client's label set are shielded from abrupt representation change. Through extensive empirical evaluations, we demonstrate that our approach can greatly alleviate this problem, especially in the most challenging federated learning settings with high heterogeneity, low participation, and large numbers of clients. 