The extraordinary properties of biological materials often result from their sophisticated hierarchical structures. Through multilevel and cross-scale structural designs, biological materials offset the weakness of their individual building blocks and enhance performance at multiple length scales to match the multifunctional needs of organisms. One essential merit of hierarchical structure is that it can optimize the interfacial features of the building blocks at different length scales, from the molecular level to the macroscale. Understanding the roles of biological material interfaces (BMIs) on the determination of properties and functions of biological materials has become a growing interdisciplinary research area in recent years. A pivotal aim of these studies is to use BMIs as inspiration for developing bioinspired and biomimetic materials and devices with advanced structures and functions. Given these considerations, this review aims to comprehensively discuss the structure-property-function relationships of BMIs in nature. We particularly focus on the discussion of BMIs and their inspired materials from mechanical and optical perspectives because these two directions are the most well-investigated and closely related. The challenges and directions of design and fabrication of BMI-inspired mechanical and optical materials are also discussed. This review is expected to garner interest from advanced material communities as well as environmental, nanotechnology, food processing, and engineering fields.