1. INTRODUCTION

2. MAIN 21-CM AND OPTICAL CATALOGS

2.1. Statistical Properties of Catalogs and Cross-identification of Sources

2.2. Velocity and Radial Distance of Sources Referred to the CMB Frame

3. HI CONTROL SAMPLES

3.1. Local Number Density Measurements for ALFALFA and SDSS Galaxies

3.2. The Low Density Environment HI Galaxy Sample

3.3. Isolated Galaxy Samples

4. UNDERLYING PROPERTIES OF GALAXIES IN THE HI CONTROL SAMPLES

4.1. Structure, Colors, and Morphology of the LDE Galaxies

4.2. Structure, Colors, and Morphology of the Members of the IG1 and IG2 Subsets

5. SUMMARY AND CONCLUSIONS