Title: Branch-Train-Merge: Embarrassingly Parallel Training of Expert Language Models. Abstract: We present Branch-Train-Merge (BTM), a communication-efficient algorithm for embarrassingly parallel training of large language models (LLMs). We show it is possible to independently train subparts of a new class of LLMs on different subsets of the data, eliminating the massive multi-node synchronization currently required to train LLMs.  BTM learns a set of independent Expert LMs (ELMs), each specialized to a different textual domain, such as scientific or legal text. These ELMs can be added and removed to update data coverage, ensembled to generalize to new domains, or averaged to collapse back to a single LM for efficient inference. New ELMs are  learned by branching from (mixtures of) ELMs in the current set, further training on new domains, and then merging the resulting models back into the set for future use. Experiments show that BTM improves in- and out-of-domain perplexities as compared to GPT-style Transformer LMs, when controlling for training cost. Through extensive analysis, we show that these results are robust to different ELM initialization schemes, but require expert domain specialization; ensembles with random data splits do not perform well. Our results suggest that aggressive parallelism could be used to efficiently scale larger LMs in future work.