The Vedas are the most ancient and sacred scriptures of the Hindu religion. The Upanishads form a very important part of the Vedas, called the "jnanakanda,"which means the portion containing the sacred knowledge. That "sacred knowledge" is the knowledge of the Brahman, the Supreme God. The nature of Brahman or Atman, though very difficult to describe in words, is the theme of the Upanishads. The Upanishads expound the vedantic philosophy according to which the inner self or Atman within a person is immortal, blissful and divine. Once a person realizes Brahman, he/she becomes Brahman himself/herself. Through thousands of years sages of India have experienced it themselves and have taught their students how to realize Brahman. There are more than two hundred Upanishads in the Vedas. Shankaracharya, the eighth century vedantic philosopher and scholar, recommended that out of these only ten Upanishads are most important. He called them "Principal Upanishads." He wrote his commentaries on those and established a sequential order in which they should be studied. The present book brings forth that sacred knowledge of the Supreme God in simple English for anybody who is willing to study it. In this book the Principal Upanishads are arranged in traditional sequence. In the beginning of each Upanishad the author discusses its content in brief and presents his own commentary where necessary.