First, spatially localizing a catalyst, either through confinement, surface attachment, or any other means previously discussed, may lead to changes in a catalyst's activity and stability. Though it is commonplace for a catalyst's activity to be accelerated by a confinement, it is possible to observe a reduction in activity upon encapsulation as well. Further, catalysts may also become more stable when confined, for example when shielded from a deactivating environment, 20,27 though the contrary is also possible. Compartmentalization of a catalyst can be regarded as essentially modifying its secondary coordination sphere or local environment, and understandably alter its activity. Such a phenomenon is well documented in the biochemical literature, as well as in work related to confining organometallic catalysts. In addition, immobilizing a catalyst to a surface support can be regarded as modifying a catalyst's primary coordination sphere, where the surface acts essentially as a ligand. Again, this may result in stark differences in activity, be it desirable or undesirable. Some of these forms of spatial control may also result in selectivity changes, which is well documented in the confinement of organometallic catalysts. At the moment, it is unfortunately difficult to predict a priori whether spatial control of a catalyst will positively or negatively (or not at all) affect a catalytic behavior.