[quote="SonCatcher, post:3, topic:215180"]
As far as I know, the celebrant is only required to wear an alb and stole. A Chasuble may be optional but certainly preferred. It's important to note this is discipline, not doctrine. In case of great need, I'm not sure any specific vestments are required.
I have often seen a priest on retreat or for daily mass use only the alb and stole.
Actually, the chasuble is required. It should be worn for all masses. The only exception is when there is a large number of concelebrants, and not enough chasubles. Other than that, even concelebrants should have chasubles. And even in the exception above, it is highly encouraged to get chasubles for them to wear.
At large (diocesan) masses, all the priests except the "main concelebrants" (technicly, there's no difference between "main concelebrants" and "general concelebrants". Usually the "Main" ones are other bishops and the rector of the cathedral) wear alb and stole, but I hear the diocese is looking into getting enough chasubles for all of them.