How to Make Text Bold in LaTeX

Why it happens

LaTeX has two kinds of font command. \textbf{...} takes an argument and bolds only that argument; \bfseries is a declaration that stays in force until the current group or environment ends. Math is set in separate math fonts, so text commands do not bold a symbol the way you expect: \textbf{x} inside $...$ gives an upright text-font x, not a bold variable. That is why math has its own bold commands.

Example

\usepackage{bm}   % preamble, only needed for \bm
% in the document:
This is \textbf{bold}, \textit{italic}, \underline{underlined}
and \texttt{monospaced}.

% Bold and italic together:
\textbf{\textit{bold italic}}

% A longer passage: \bfseries lasts until the group ends
{\bfseries This sentence is bold.

So is this whole paragraph.}

% Bold in math:
$\mathbf{v} = (v_1, v_2)$ and $\bm{\alpha} + \bm{\beta}$

\textbf vs \bfseries vs \bf

Use \textbf{...} for a word or a phrase. It takes one argument, so it cannot run on by accident. It also cannot hold more than one paragraph: a blank line inside \textbf{...} stops the compile with "Paragraph ended before \text@command was complete".

Use \bfseries for anything longer, and inside your own commands and environments: {\bfseries ...} or \begin{center}\bfseries ... \end{center}. It bolds everything up to the closing brace or the \end.

Avoid \bf. It is a LaTeX 2.09 command that the standard classes keep only for old documents, and it resets the font before switching, so {\bf\it text} comes out italic but not bold, while \textbf{\textit{text}} gives real bold italic. The KOMA-Script classes (scrartcl, scrreprt, scrbook) reject it with "Class scrartcl Error: undefined old font command". The same is true of \it, \rm, \tt and \sc.

Bold in math: \mathbf, \bm and \boldsymbol

\mathbf{v} gives upright bold Roman letters and digits, the usual style for vectors and matrices. It has no effect on lowercase Greek: \mathbf{\alpha} prints an ordinary alpha, with no error to tell you.

For bold Greek letters and bold italic variables, load \usepackage{bm} and write \bm{\alpha} or \bm{x}. amsmath provides \boldsymbol{...}, which does the same job for most symbols.

To set a whole formula in bold, for example inside a bold heading, put \boldmath before the math starts: {\boldmath $f(x) = x^2$}. Inside math it does nothing, and LaTeX warns "Command \boldmath invalid in math mode". The reverse mistake, \mathbf in ordinary text, stops with "LaTeX Error: \mathbf allowed only in math mode"; use \textbf there.

\usepackage{amsmath}
\usepackage{bm}
% in the document:
$\mathbf{A}\mathbf{x} = \mathbf{b}$     % bold upright vectors
$\bm{\mu} = \bm{\Sigma}^{-1}\bm{x}$     % bold Greek and italic
{\boldmath $f(x) = x^2$}              % a whole formula in bold

Bold in tables, headings and the title

Section headings are already bold, so \textbf inside \section{...} changes nothing you can see. To take the bold off part of a heading, use \textmd{...}, as in \section{Results \textmd{(draft)}}. To restyle every heading, use the titlesec package rather than editing each one.

The title printed by \maketitle in the article class is large but not bold. Write \title{\textbf{Your title}} if you want it bold.

In a table, bold one cell with \textbf{...}. To bold a whole column, load the array package and put >{\bfseries} before the column letter.

\usepackage{array}
\usepackage{booktabs}
% in the document:
\begin{tabular}{>{\bfseries}l r}
  \toprule
  Model    & \textbf{Accuracy} \\
  \midrule
  Baseline & 71.2 \\
  Ours     & \textbf{84.5} \\
  \bottomrule
\end{tabular}

When the bold does not show up

Bold monospace: with the default Computer Modern fonts, \texttt{\textbf{code}} prints ordinary typewriter text. Computer Modern has no bold typewriter face, and LaTeX substitutes the regular one without a warning. Add \usepackage[T1]{fontenc} and \usepackage{lmodern}, which include a bold monospace.

Bold small caps: \textbf{\textsc{...}} has no matching face in Computer Modern or Latin Modern either. Here the log does tell you: "LaTeX Font Warning: Font shape `OT1/cmr/bx/sc' undefined", and LaTeX falls back to ordinary bold, without the small capitals. Only a font family that has bold small caps can fix that.

"Undefined control sequence" on a line with \bm means the bm package is not loaded.