\addcontentsline{toc}{chapter}{Introduction}
\chapter*{Introduction}
An extensive search for top quarks has been performed at
electron-positron and hadron colliders for more than a decade.  First
evidence for top quark production in proton-antiproton collisions has
been announced by the CDF collaboration in the spring of 1994.  After
collecting  more luminosity subsequently both the CDF and the D0
experiments presented the definite analysis \cite{Sinerva}
 which demonstrated not only the existence of the
anti\-cipated quark but at the same time also provided a kinematic
determination of the top quark mass around 180 GeV and a production
cross section consistent with the QCD predictions.  The mass value is
in perfect agreement with the indirect mass determinations based on
precision measurements 
%\cite{bardin,HollikBrus,Swartz,Kniehl,Sirlin,degrassi} 
[2-7]
of the
electroweak 
parameters in \ep annihilation and in lepton-nucleon scattering.
Exploiting the quadratic top mass dependence of radiative corrections
an indirect mass measurement of 180 GeV with a present uncertainty of
roughly $20$  GeV  has been achieved.  

The top quark completes the fermionic spectrum of the Standard Model.
 All its properties are uniquely fixed after the mass has been
determined.  However, as a consequence of its large mass and decay
rate it will behave markedly different compared to the remaining five
lighter quarks.  

It is not just the obvious aim for completion which raises the
interest in the top quark.  With its mass comparable to the scale of
the electroweak symmetry breaking it is plausible that top quark
studies could provide  an important clue for the understanding of
fermion mass generation and the pattern of Yukawa couplings.  In fact,
it has been suggested that a top quark condensate could even be
responsible for the mechanism of spontaneous symmetry breaking
\cite{Hill}. 

These lectures will be mainly concerned with top quark phenomenology
within the framework of the Standard Model (SM).  
The precise understanding of its
production and decay constitutes the basis of any search for
deviations or physics beyond the SM. 

The properties of the top quark will be covered in chapter 1.  Direct
and indirect determinations of its decay rates, decay distributions
including QCD and electroweak corrections and decay modes predicted in
supersymmetric extensions will be discussed.  Top quark production at
hadron colliders will be the subject of chapter 2.  The production
cross section and momentum distributions are important ingredients in
any of the present analysis.   An alternative reaction, namely top
quark production through $W$-$b$-fusion allows to determine the
$W$-$b$-$\bar  t$ coupling and thus indirectly the top quark decay rate.

Perspectives for top studies at a future \ep collider will be
presented in chapter 3.  An accurate determination of the top quark
mass and its width to better than 1 GeV with a relative accuracy of
about 10\% seems feasible, and the electroweak couplings of the top
quark could be precisely measured with the help of polarized beams.
Of particular interest is the interplay between the large top quark
decay rate and the binding through the QCD potential which will be
also covered in chapter 3.
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