Upanishads are the fountain head of Indian cultural and spiritual knowledge as contemplated and visualized by our ancient sages and seers. Initially they were handed down for generations as oral tradition through preceptors - disciples in gurukula tradition as secret knowledge (raja guhyam), later was documented in Sanskrit language, one of the oldest language in the world. They were called as Vedanta as they came at the end of the Vedas, which are a compendium of ancient cultural, social and spiritual thoughts. Hence upanishadic thoughts are universal in their appeal and catholic in approach. They belong to the whole world. A serious study and practical implementation of upanishadic thoughts by any human being will lead one to physical, mental, intellectual and spiritual happiness in this world and a person can escape the cycles of birth and death leading to final liberation (moksha). The Upanishads speak both of 'within' and 'without' of things. One of the most important concept embedded in the Upanishads is the Consciousness. It is called as chit, prajna, chaitanya or Brahman. This paper reviews the concept of consciousness as explained in the principle ten Upanishads. It also brings about the types of consciousness as elaborated in Mandukyopanishad. The four states of consciousness namely waking (jagrat), dream (svapnam), deep sleep (sushupti) and turiya or fourth are analysed here as per Gaudapada’s Karika on Mandukyopanishad. It also brings about the transcendental consciousness (samadhi) as experienced by Sri Ramakrishna Paramahamsa and the final spiritual goal of seekers.