Hi, Having now spent some time porting my pqs library http://www.servocomm.freeserve.co.uk/Cpp/physical_quantity/index.html over to boost, I am currently of the opinion that using mpl is not really practical for use with the pqs library. The main issue is regarding the large amount of types created behind the scenes by the mpl library. This is not acceptable in pqs because pqs uses a large number of parameters per quantity. The VC7.1 compiler is now frequently coming up with an fatal error message 'out of keys' during compilation of tests. For comparison this wasnt a noiceable problem in the non-boost version Obviously for the library to be remotely useable I need to be as sparing as possible with compiler resouces, allowing the user as much room to work as possible. In order for the pqs library to be practical I need to use a family of types for the dimension parameters that is designed with great care with respect to the total number of types created behind the scenes. Even then I need to acknowledge the issue of use of compiler resources by the library in the documentation. An example niggle regarding the mpl arithmetic operations , that they each use 5 parameters for e.g. addition whereas minimally only 2 are required. This also adds to the bloat for example. This is a particlar problem in the pqs library. Another issue with mpl concerns the fact that mpl doesnt deal in types. This is a problem when trying to register types in conjunction with BoostTypeof . .. What types to register ? (This is only a problem because the lack of an inbuilt typeof operator in C++). I'll certainly leave the current version of boost::pqs around (pqs_3_0_1 now in the vault in Physical Quantities Units directory) . I am aware that the situation could be improved slighltly for example by not allowing conversions from rational types to integral types, though the problem would still remain in lesser form) I hope that pqs-3-0-1 will answer the question why I have opted not to use mpl in the implementation in the next version. regards Andy Little