The shortest of the principal Upanishads, opening the Shukla Yajurveda with a single teaching: that the divine pervades everything, and true renunciation means enjoying the world without grasping at it.
शुक्ल यजुर्वेद के अंतर्गत · within the Shukla Yajurveda
In just eighteen verses, the Isha Upanishad opens with 'Ishavasyam idam sarvam,' all this is pervaded by the Lord, then argues that one should live a full worldly life through renunciation rather than by rejecting the world outright, desiring nothing that is not freely given.
Tradition places the Isha Upanishad first among the principal Upanishads in most numbered lists, and unlike most others it sits directly within the Yajurveda's own mantra portion (samhita) rather than being appended to it later — a placement the tradition reads as marking it especially close to the Veda's own voice.