Title: Intepreting & Improving Pretrained Language Models: A Probabilistic Conceptual Approach. Abstract: Pretrained Language Models (PLMs) such as BERT and its variants have achieved remarkable success in natural language processing. To date, the interpretability of PLMs has primarily relied on the attention weights in their self-attention layers. However, these attention weights only provide word-level interpretations, failing to capture higher-level structures, and are therefore lacking in readability and intuitiveness. In this paper, we propose a hierarchical Bayesian deep learning model, dubbed continuous latent Dirichlet allocation (CLDA), to go beyond word-level interpretations and provide concept-level interpretations. Our CLDA is compatible with any attention-based PLMs and can work as either (1) an interpreter which interprets model predictions at the concept level without any performance sacrifice or (2) a regulator which is jointly trained with PLMs during finetuning to further improve performance. Experimental results on various benchmark datasets show that our approach can successfully provide conceptual interpretation and performance improvement for state-of-the-art PLMs. 