How to Place Figures Side by Side in LaTeX

Why it happens

A figure environment is one float with one number. subcaption lets you put several boxes inside it, each a fixed fraction of the line width with its own caption and label. LaTeX places those boxes the way it places words on a line, so their widths have to add up to less than \linewidth, and a blank line between two of them starts a new row.

Example

\usepackage{graphicx}
\usepackage{subcaption}
% in the document (example-image-a ships with TeX Live; use your own files):
\begin{figure}
  \centering
  \begin{subfigure}[b]{0.48\textwidth}
    \centering
    \includegraphics[width=\linewidth]{example-image-a}
    \caption{First}\label{fig:a}
  \end{subfigure}\hfill
  \begin{subfigure}[b]{0.48\textwidth}
    \centering
    \includegraphics[width=\linewidth]{example-image-b}
    \caption{Second}\label{fig:b}
  \end{subfigure}
  \caption{Two results side by side}\label{fig:both}
\end{figure}

Widths, spacing and alignment

The widths, plus any space between the subfigures, must fit in \linewidth. Two at 0.48\textwidth with \hfill between them fit. Two at 0.5\textwidth do not, because the line break in your source between \end{subfigure} and the next \begin{subfigure} counts as a space: LaTeX reports an "Overfull \hbox" a few points wide, and the pair sticks into the margin. End the line with % or use smaller widths. Widths that add up to well over \linewidth do the same, with a bigger number.

The optional argument sets the vertical alignment when the images differ in height: [b] lines up the bottoms, so the captions sit level, [t] the tops, and the default [c] the centres. \centering inside each subfigure centres an image narrower than its box.

A 2x2 grid

For two rows, leave a blank line after the second subfigure, then add space between the rows with \medskip or \vspace{1em}. The letters continue across the rows: (a), (b), (c), (d).

\usepackage{graphicx}
\usepackage{subcaption}
% in the document:
\begin{figure}
  \centering
  \begin{subfigure}{0.45\textwidth}
    \includegraphics[width=\linewidth]{example-image-a}
    \caption{Top left}
  \end{subfigure}\hfill
  \begin{subfigure}{0.45\textwidth}
    \includegraphics[width=\linewidth]{example-image-b}
    \caption{Top right}
  \end{subfigure}

  \medskip
  \begin{subfigure}{0.45\textwidth}
    \includegraphics[width=\linewidth]{example-image-c}
    \caption{Bottom left}
  \end{subfigure}\hfill
  \begin{subfigure}{0.45\textwidth}
    \includegraphics[width=\linewidth]{example-image}
    \caption{Bottom right}
  \end{subfigure}
  \caption{Four panels}
\end{figure}

Two separate figures side by side

If the images are separate figures, numbered Figure 1 and Figure 2 rather than 1a and 1b, put two minipage environments inside one figure, each with an ordinary \caption. No extra package is needed.

\usepackage{graphicx}
% in the document:
\begin{figure}
  \centering
  \begin{minipage}{0.48\textwidth}
    \centering
    \includegraphics[width=\linewidth]{example-image-a}
    \caption{Before}\label{fig:before}
  \end{minipage}\hfill
  \begin{minipage}{0.48\textwidth}
    \centering
    \includegraphics[width=\linewidth]{example-image-b}
    \caption{After}\label{fig:after}
  \end{minipage}
\end{figure}

References and the errors you will meet

\ref{fig:a} prints 1a, and \subref{fig:a} prints only the letter, a, for sentences such as "panels a and b". With cleveref, \cref{fig:a} prints "fig. 1a" and \Cref prints "Figure 1a".

"Environment subfigure undefined": subcaption is not loaded.

"Missing number, treated as zero" followed by "Illegal unit of measure (pt inserted)" on a \begin{subfigure} line: the width argument is missing, or the old subfigure package is loaded as well and has taken over the name. Load subcaption alone; subfig and subfigure are older packages that conflict with it.

Subfigures stacked one above the other: there is a blank line between them.