How to Add Footnotes in LaTeX

Why it happens

Footnote text is placed by the page builder, which only sees material in the main flow of the page. A tabular cell or a float is built as a separate box first, so a \footnote there prints its mark but its text never reaches the bottom of the page, and LaTeX gives no warning. minipage is the exception: it collects its own footnotes and prints them underneath, lettered a, b, c.

Example

This claim needs a source.\footnote{Smith, 2020, p.~4.}
Most English styles put the mark after the punctuation.

% In a table: the mark in the cell, the text after the table
\begin{table}[h]
  \centering
  \begin{tabular}{lr}
    Revenue\footnotemark & 120 \\
  \end{tabular}
  \caption{Results}
\end{table}
\footnotetext{Before tax.}

Footnotes in tables: three ways

\footnotemark and \footnotetext give an ordinary page footnote. \footnotemark steps the counter and prints the mark; \footnotetext{...} after \end{table} prints the text with the current number. For two marks in one table, step back before the first text: \addtocounter{footnote}{-1}\footnotetext{...}\stepcounter{footnote}\footnotetext{...}. If the table floats to another page, the note stays on the page where \footnotetext was written, so check the result, or keep the table in place with [h] or with [H] from the float package.

threeparttable puts the notes directly under the table, marked a, b or *, the usual layout for table notes. The tablefootnote package provides \tablefootnote{...}, which works like \footnote inside a table float.

A tabular inside a minipage gets its footnotes printed under the minipage, with letters, and needs no package.

\usepackage{threeparttable}
% in the document:
\begin{table}[h]
  \centering
  \begin{threeparttable}
    \caption{Quarterly results}
    \begin{tabular}{lrr}
      Item             & Q1 (USD million) & Q2 (USD million) \\
      Revenue\tnote{a} & 120              & 135              \\
      Costs\tnote{b}   &  80              &  84              \\
    \end{tabular}
    \begin{tablenotes}
      \item[a] Before tax.
      \item[b] Including staff costs.
    \end{tablenotes}
  \end{threeparttable}
\end{table}

Symbols, numbering and placement

Footnotes run 1, 2, 3 through the whole document in article, and restart with each chapter in report and book. \renewcommand{\thefootnote}{\fnsymbol{footnote}} switches to *, †, ‡ and so on. \fnsymbol has only nine symbols, and a tenth note stops with "LaTeX Error: Counter too large", so reset the counter with \setcounter{footnote}{0} where it makes sense.

The footmisc package adds options: perpage restarts the numbering on each page (compile twice so the numbers settle), symbol uses symbols, and bottom keeps footnotes at the foot of the page. Without bottom, a short page in a one-sided document puts its footnotes right under the last line of text instead.

\usepackage[perpage,symbol]{footmisc}
% in the document:
A first note\footnote{Marked with an asterisk.}
and a second\footnote{Marked with a dagger.}.

Footnotes in headings, captions and the title

A \footnote inside \section{...} breaks the compile with "Argument of \@sect has an extra }", because the heading text is also written to the table of contents and the page headers. Give the heading a short form without the note: \section[Methods]{Methods\footnote{...}}.

Captions sit inside floats, so a footnote there is lost for the reason above. Use \footnotemark in the caption, give the caption a short form for the list of figures, and put \footnotetext after the float.

For the title and the authors, use \thanks{...} inside \title or \author. \maketitle prints those notes with symbols, and numbering in the text still starts at 1.

\documentclass{article}
\title{A Study of Things\thanks{Funded by grant 123.}}
\author{Jane Smith\thanks{University of Somewhere.}}
\begin{document}
\maketitle
\section[Methods]{Methods\footnote{Adapted from earlier work.}}
Body text with a note.\footnote{This one is numbered 2.}
\end{document}

Reusing a footnote and other details

To point to the same note twice, put a \label inside the footnote and write \footref{...} where the mark should appear again. \footref is part of LaTeX itself since the June 2021 release, and footmisc provides it for older systems.

A space before \footnote shows up as a gap before the mark. Write the command directly after the word, and if it has to start on a new line, end the previous line with %.

Long footnotes can split across pages. To keep each one on a single page, set \interfootnotelinepenalty=10000 in the preamble.

Text with a note\footnote{A remark that applies twice.\label{fn:shared}}
and a second mention.\footref{fn:shared}