Paper has played a fundamental role in human history and remains essential today. The unique properties of paper and its responsible production and consumption contribute to maintaining its continued relevance. In addition, most paper is biodegradable and one of the most recycled materials in the world. Paper grades refer to classifications of paper and board products according to their quality, use, appearance, performance, or other relevant characteristics or based on a combination of factors. Some paper grades are standardized in the industry, each designed to fulfill specific functions. This chapter summarizes the main physical, optical, mechanical, thermal, and electrical properties of paper and paperboard. The chapter also specified which properties play a fundamental role according to its application.