Blender 2.76 - I've always had problems with this, even in 2.49 (but it wasn't so bad there -sometimes it worked - I think?). I must be doing something wrong.

I want the red 'Basin' material on the unselected faces, and the green 'Surface' material on the selected face. I start with the red material applied to everything. I select the face I want to be green. I open the material list...

...and select the green material, expecting nothing to happen in the 3D View pane. I expected to then click the 'Assign' button to assign the green material to the selected face.

But as soon as I clicked the green material from the drop down it's immediately applied to everything.

I'm obviously misunderstanding something very basic.

Can somebody please enlighten me ? :)

Hope the attached picture helps out.

Thanks, it does. Comparing your screenshot to mine I was trying to select my material from the "Browse Material to be Linked" drop-down,when what I should have been doing was adding a new material slot using the '+' button you pointed out, and then adding a new material to that empty slot

(I don't think I've ever really understood the datablocks, slots and linking... I may be back)

I'd be interested to hear what others say about this, but I've found that if I'm dealing with objects where different materials share an edge, it's best to:

1. go to face mode first,
2. select the face(s) to receive the new material and then
3. assign the new material to the selected face(s).

And I always seem to get better results if I UV-map first (but I guess that goes without saying... )

^That's the way I do it. That way I have a visual of what areas haven't been assigned a material as they'll be the standard grey-shaded.