  A covariant calculus for the construction of effective string theories is
developed. Effective string theory, describing quantum string-like excitations
in arbitrary dimension, has in the past been constructed using the principles
of conformal field theory, but not in a systematic way. Using the freedom of
choice of field definition, a particular field definition is made in a
systematic way to allow an explicit construction of effective string theories
with manifest exact conformal symmetry. The impossibility of a manifestly
invariant description of the Polchinski-Strominger Lagrangian is demonstrated
and its meaning is explained.
