This is a draft of an article for Annual Reviews of Astronomy and Astrophysics

1. Introduction

1.1 What is a Brown Dwarf?

2. The Search for Brown Dwarfs

2.1 A Brief History of the Searches

2.2 The First Incontrovertible Brown Dwarf: Gl 229B

3. Distinguishing YOUNG Brown Dwarfs from stars

3.1 The Principle of the Lithium Test

3.2 The Lithium Test in Young Clusters

3.2.1 The Age Scale for Young Clusters

3.3 The Lithium Test in the Field

3.4 Brown Dwarfs in Star Forming Regions

4. Objects Cooler than M Stars

4.1 Field Brown Dwarfs and VLM Stars

4.2 The Atmospheres of Very Cool Objects

4.3 Definition of the L Spectral Class

4.4 The Methane Dwarfs

4.4.1 The Atmosphere of Gl 229B

4.5 Rotation and Activity in VLM Objects

5. Brown Dwarfs in Binary Systems

5.1 Visible Brown Dwarf Companions

5.2 Radial Velocity Brown Dwarf Candidates

5.3 Double Brown Dwarfs

6. The Substellar Mass Function

6.1 The Mass Function from Clusters

6.2 The Mass Function for Binaries

6.3 The Mass Function in the Field

Bibliography