Hydrogen-Deficient Stars: An Introduction

We describe the discovery, classification and statistics of hydrogen-deficient stars in the Galaxy and beyond. The stars are divided into (i) massive / young star evolution, (ii) low-mass supergiants, (iii) hot subdwarfs, (iv) central stars of planetary nebulae, and (v) white dwarfs. We introduce some of the challenges presented by these stars for understanding stellar structure and evolution.