Add bold enumerate items

If you want the whole item bold, you can write:

\begin{enumerate}\bfseries
\item The firs item
\item The second item
\end{enumerate}

If you want the number to be bold, you can do it by using the enumitem package and setting:

\begin{enumerate}[label=\textbf{\arabic*})]
\item The firs item
\item The second item
\end{enumerate}

Check the enumitem package documentation for more info.

This is in response to this comment by Raffi Khatchadourian on the accepted answer: "I see a problem here: this answer is redefining the numbering style completely instead of enhancing it, i.e., making the number part bold."

If you would like to avoid this problem with the top two answers, then there are two different solutions, one local and one global, using the enumitem package.

The idea for both is to use the font= key for enumitem instead of the label= key.

Local: Every time you want an individual enumerate environment with bold numbers/letters/numerals (whatever the setting is), just use:

\begin{enumerate}[font=\bfseries]
*your code here*
\end{enumerate}

Global: If you know in advance that you will want every enumerate environment to have bold numbers/letters/numerals (whatever the setting is), then put the following in the document's preamble:

\setlist[enumerate]{font=\bfseries}

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Source: I read the documentation for enumitem, and both of the above solutions worked for me using TeXstudio and XeLaTeX compiler.

If you mean having the item number in bold, you can do it like this:

\documentclass{article}
\usepackage{enumitem}
\begin{document}
    \begin{enumerate}[label=\textbf{\arabic*}]
        \item My first item.
        \item My second item.
    \end{enumerate}
\end{document}