  Let $G=G(K)$ be a simple algebraic group defined over an algebraically closed
field $K$ of characteristic $p>0$. A subgroup $X$ of $G$ is said to be
$G$-completely reducible if, whenever it is contained in a parabolic subgroup
of $G$, it is contained in a Levi subgroup of that parabolic. A subgroup $X$ of
$G$ is said to be $G$-irreducible if $X$ is in no parabolic subgroup of $G$;
and $G$-reducible if it is in some parabolic of $G$. In this thesis, we
consider the case that $G$ is of exceptional type. When $G$ is of type $G_2$ we
find all conjugacy classes of closed, connected, reductive subgroups of $G$.
When $G$ is of type $F_4$ we find all conjugacy classes of closed, connected,
reductive $G$-reducible subgroups $X$ of $G$. Thus we also find all
non-$G$-completely reducible closed, connected, reductive subgroups of $G$.
When $X$ is closed, connected and simple of rank at least two, we find all
conjugacy classes of $G$-irreducible subgroups $X$ of $G$. Together with the
work of Amende in [Ame05] classifying irreducible subgroups of type $A_1$ this
gives a complete classification of the simple subgroups of $G$.
  Amongst the classification of subgroups of $G=F_4(K)$ we find infinite
collections of subgroups $X$ of $G$ which are maximal amongst all reductive
subgroups of $G$ but not maximal subgroups of $G$; thus they are not contained
in any maximal reductive subgroup of $G$. The connected, semisimple subgroups
contained in no maximal reductive subgroup of $G$ are of type $A_1$ when $p=3$
and of semisimple type $A_1^2$ or $A_1$ when $p=2$. Some of those which occur
when $p=2$ act indecomposably on the 26-dimensional irreducible representation
of $G$.
  We also use this classification to find all subgroups of $G=F_4$ which are
generated by short root elements of $G$, by utilising and extending the results
of [LS94].
