There are days when you can't decide whether or not to wear a base layer and you waste time with a debate you can't answer. Craft's Cool Mesh Superlight Sleeveless Base Layer answers the question for you. Wear it.
This is as minimal as a base layer can be. The top is made of Craft's C017 lightweight mesh fabric. Big holes woven with a hexachannel fiber gives plenty of opportunity for cooling with just a smidge of insulation. The material is also sufficiently elastic that you wear this for the advantages of a body-hugging fit without ever feeling encased.
Even though it's sleeveless, this base layer has gotten the same attention from Craft's ergonomic experts. The paneling wraps around you, with generous arm holes and seams that wrap around you.
The Craft Cool Mesh Superlight Base Layer comes in three colors and is available in sizes from X-Small to XX-Large.