Neb,

I assume by "aliasing" you are refererring to some pixellation somewhere in your scene, otherwise known as "jaggies"? So you want anti-aliasing?

If you're getting jaggies in the shadow, try setting the light's Shadow Blur Radius (in Properties) - this softens the edge of the shadow.

If it isn't micro-pixellated, but rather the edges seem to be squaring off (bigger squares than a pixel) then it's because your shadow map size is too small. Go to the light's "Map Size" (in Parameters). The default map size is way too small. Go for at least 800, maybe 1200 or 1600. Shadow  maps are actually images that get automatically generated and then used as a mask on the light. When this image is too low resolution, you can see it's pixels. Raising the shadow map size makes the shadow image pixels smaller and therefore more accurate shadows result. Be prepared to wait a lot longer for the shadow map calculation. I suggest you enable the menu item "Render/Reuse Shadow Maps". This will only calculate the shadows once. If you move things, click "Render/Clear Shadow Maps" to get it to recalculate.

If you're talking about general jaggies everywhere, go to Render Settings (I always use manual) and make sure the "Pixel samples" is bigger than 1. I usually use 3, but sometimes 5. If things are blurry as a result, lower the "Min shading rate" - I usually use .5, but in extreme cases I use .2. If you ever render a textured object and the texture looks blurry and you've loaded it at full size (Max texture size is at least as large) then the "Min shading rate" reduction will bring out the details.

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