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Studies of collisions and mergers among galaxies were brought into the mainstream of extragalactic research through the classic paper of Toomre \& Toomre (1972, ApJ, 178, 623). I will review some of the ideas presented in that paper and describe some of the related research that has followed in the subsequent 20 years. In particular, I will focus on recent developments in the areas of:
\begin{itemize}
\item Numerical simulations of encounters:~ theory meets observation.
\item The galaxy merger rate:~ $1+1=1$ .
\item Building ellipticals from mergers of spirals:~ Sp $+$ Sp = E ?
\end{itemize}
In each of these areas, very active research programs have been pursued over the past two decades. Many of the results of these investigations have validated the longevity and robustness of the insights and ideas presented in the Toomre \& Toomre study.
