Shape fills such as solid colors, gradients, pictures, and textures can have a transparency attribute that lets you reduce the opacity of a fill so that the slide object or background behind shows through. Transparency is calculated in percentages and you can change its value all the way from 0 to 100%. Some fill options, such as pattern and slide background have no transparency options.
Learn how to apply transparency to shape fills in PowerPoint 2010 for Windows.

