This chapter discusses theoretical models that describe the physical processes
of pion decays. Discussions on the weak force as it relates to the Standard
Model (SM) and the conserved vector current hypothesis (CVC) are presented.
First order calculations of the [lambda]( p ^{+} ->
p ^{0} e^{+} n ) / [lambda]( p ^{+} ->
µ^{+} n ) and [lambda]( p ^{+} -> e^{+}
n ) / [lambda]( p ^{+} -> µ^{+} n ) branching
ratios are presented with discussions on second order corrections.