One chapter without numeration

For the standard classes (article, report, book) you can use the starred version of \chapter which suppresses the number but also the TOC entry, so you hav to add it manually with \addcontentsline

\documentclass{report}

\begin{document}
\tableofcontents

\chapter*{Introduction}
\addcontentsline{toc}{chapter}{Introduction}  
\chapter{Chapter 1}  
\chapter{Chapter 2} 
\end{document}

or with KOMA-Script (scrartcl,scrreprt,scrbook) which provides \addchap for this case (and \addsec of sections):

\documentclass{scrreprt}

\begin{document}
\tableofcontents

\addchap{Introduction}
\chapter{Chapter 1}  
\chapter{Chapter 2} 
\end{document}

I don’t know about memoire’s behavior when using \chapter

MWCLS supports \chapter* and you don't have to do anything more, it will end in TOC, because Polish rules require it.