  We present the causal construction of perturbative Yang-Mills theories in
four(3+1)-dimensional space-time. We work with free quantum fields throughout.
The inductive causal method by Epstein and Glaser leads directly to a finite
perturbation series and does not rely on an intermediary regularization of the
theory. The causal method naturally separates the physical infrared problem of
massless theories from ultraviolet-sensitive features like normalizability by
regarding the distributional character of the S-matrix. We prove the
normalizability of the Yang-Mills theory with fermionic matter fields and study
the discrete symmetry transformations in the causal formalism. We introduce a
definition of nonabelian gauge invariance which only involves the free
asymptotic field operators and give mathematically rigorous and conceptually
simple proofs of nonabelian gauge invariance and of the physical unitarity of
the S-matrix in all orders of perturbation theory.
