Arnold 5.0+

The old Standard shader has been replaced with the Standard Surface shader. The old Standard shader is still shipped with Arnold as a legacy shader, for compatibility with old scenes. However, it will be removed in a future release.

If all of the individual weights and colors are less than or equal to 1.0, then the Standard Surface shader is energy conserving. Unlike the old Standard shader, you don't need to worry about the sum of weights being less than 1.0 or manually enable Fresnel, which is always enabled.

The Specular Roughness affects both specular reflection and refraction. There is also an Extra Roughness parameter in Transmission to add some additional roughness for refraction. You can, however, use Coat to create a rough reflection layer over a sharp refraction.

GGX is now the default model for the Standard Surface shader.

You can use the Specular IOR attribute in the same way to handle transparent materials such as glass, diamond, etc.

Enabling Thin Walled and setting Subsurface Weight to, for example, 0.5 to have half the light reflected and half transmitted.

Using very high IOR values can look quite similar to metalness. It looks the same if you set base color to the specular color and the specular color to black. The difference is that you get an extra reflection at the edges, with the specular color controlling the edge tint. The metal Fresnel works the same as in the Complex IOR shader, with the artistic parameters.

You should use the Subsurface parameter of the Standard Surface shader. It uses the high-quality empirical SSS profile, which preserves sharp geometric and bump detail without the need for three layers of varying depth, as the old skin shader did.

Use the new Standard Hair shader instead. The old Hair shader is shipped with Arnold as a legacy shader, for compatibility with old scenes. However, it will be removed in a future release.

The hair shader does not have a "uvset" parameter at all. Rather all image and procedural texture nodes have a "uvset" parameter instead so that they can pick up the appropriate texture coordinates there.

Use the new Standard Volume shader instead. The old Volume Collector shader is shipped with Arnold as a legacy shader, for compatibility with old scenes. However, it will be removed in a future release.

Information about OSL shaders can be found here.

The Shadow Catcher shader is now called the Shadow Matte shader.