  We consider the effects of new light species on the Cosmic Microwave
Background. In the massless limit, these effects can be parameterized in terms
of a single number, the relativistic degrees of freedom. We perform a thorough
survey of natural, minimal models containing new light species and numerically
calculate the precise contribution of each of these models to this number in
the framework of effective field theory. After reviewing the relevant details
of early universe thermodynamics, we provide a map between the parameters of
any particular theory and the predicted effective number of degrees of freedom.
We then use this map to interpret the recent results from the Cosmic Microwave
Background survey done by the Planck satellite. Using this data, we present new
constraints on the parameter space of several models containing new light
species. Future measurements of the Cosmic Microwave Background can be used
with this map to further constrain the parameter space of all such models.
