Goal: Gradient that fades from “transparent > color” or “transparent > white”
Software: Adobe Illustrator CS3

  1. Draw rectangle, fill with solid color (or solid white).
  2. With rectangle still active, choose Transparency > “Make Opacity Mask.”
  3. Click on mask in the Transparency palette (it’s the black square on the right right side of the Transparency palette); rectangle will disappear (this is normal). Make sure Clip check-box is ticked.
  4. Draw a second rectangle on top of the first rectangle and fill it with a “white > black” gradient.

Original instructional article: Transparent Gradients in Adobe Illustrator