Secondary radiation of hadrons from a tau pair produced
in electron positron collisions may constitute an
important obstacle for precision measurements of
the production cross section and of branching ratios.
The rate for real and virtual radiation is calculated
and various distributions are presented.
For Z decays a comprehensive analysis is performed which
incorporates real and virtual radiation of
leptons. The corresponding results are also given for
primary electron and muon pairs.
Compact analytical formulae are presented for
entirely leptonic configurations.
Measurements of $Z$ partial decay rates which eliminate
all hadron and lepton radiation are about 0.3\% to 0.4\%
lower than totally inclusive measurements, a consequence of
the ${\cal O}(\alpha^2)$ negative virtual corrections which
are enhanced by the third power of a large logarithm.
