user56027: The STM32YYXX chip series is not defined in boards.txt? How can I solve this? I am using Windows 10 with Arduino IDE 1.6.6

user6936: Overview
Following the guide Getting Started on the STM32 page on Github: Arduino core support for STM32 based boards:

This repo is available as a package usable with Arduino Boards
  Manager.
Use this link in the "Additional Boards Managers URLs" field:
https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json
For full instructions on using the "Boards Manager", see the Getting
  Started page.

Thanks to Juraj's comment for the tip 

this is not the current STM boards package. ST Microelectronic took over the development github.com/stm32duino/Arduino_Core_STM32 stm32duino.com/viewtopic.php?f=16&t=4671

Step by Step
The process is as follows (screenshots from OS X, but on Windows/Linux the process is the same):
Adding the URL in the preferences dialog (one URL per line)



Boards Manager menu



Boards Manager dialog



Boards Manager dialog - STM32 Core Boards installed



Boards Manager dialog - Boards installing



Boards Manager dialog - Boards installed



Here are the newly installed boards in the menu



As a comparison, these are the boards installed using the deprecated manual process described below:




Deprecated method

From STM32 - installation

Prerequisites
If your STM32 board does not have a bootloader yet and you want to be able to program it over USB, you'll have to upload
  the STM32duino bootloader to flash memory. See Burning the
  bootloader.
Arduino and Arduino_STM32

Download and install the official Arduino IDE. Versions 1.6.10 till 1.8.5 have been reported to work fine.
  
  
If you already have the Arduino IDE installed and it's a newer version, see the Boards Manager package method of installation.
  
  
Note: this method is deprecated and not supported in forum!


Run Arduino IDE, and open the menu Tools -> Boards -> Boards Manager. Install the "Arduino SAM Boards" package (includes the
  Arduino Due) from the list of available boards. This installs compiler
  support for ARM Cortex-M3.
Download a zip file containing the Arduino STM32 files from here.
Unzip the content of the zip file, and place the Arduino_STM32 folder to [Arduino sketches folder]/[hardware]/[Arduino_STM32].
  Create the 'hardware' folder if it does not exist yet.

Example how the path should look like:
  C:\Users\<user>\Documents\Arduino\hardware\Arduino_STM32


As an aside, for anyone using OS X, from Installation on OSX

Installation on OSX

Arduino_STM32 folder must be placed inside
  ~/Documents/Arduino/hardware (note. if the hardware folder doesn't
  exist you will need to create it) 
So you should get ~/Documents/Arduino/hardware/Arduino_STM32
Note.  DFU util binaries
  have now been added to the repo, in tools/macosx/dfu-util, so there is
  no longer a need to install Homebrew to then install dfu-util, or to
  compile dfu-util from source.
  



56027: <p>The STM32YYXX chip series is not defined in <code>boards.txt</code>? How can I solve this? I am using Windows 10 with Arduino IDE 1.6.6</p>
 6936: <h3>Overview</h3>

<p>Following the guide <a href="https://github.com/stm32duino/Arduino_Core_STM32#getting-started" rel="nofollow noreferrer">Getting Started</a> on the STM32 page on Github: <a href="https://github.com/stm32duino/Arduino_Core_STM32" rel="nofollow noreferrer">Arduino core support for STM32 based boards</a>:</p>

<blockquote>
  <p>This repo is available as a package usable with <a href="https://www.arduino.cc/en/guide/cores" rel="nofollow noreferrer">Arduino Boards
  Manager</a>.</p>
  
  <p>Use this link in the "Additional Boards Managers URLs" field:</p>
  
  <p><a href="https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json" rel="nofollow noreferrer"><code>https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json</code></a></p>
  
  <p>For full instructions on using the "Boards Manager", see the <a href="https://github.com/stm32duino/wiki/wiki/Getting-Started" rel="nofollow noreferrer">Getting
  Started</a> page.</p>
</blockquote>

<p><sub>Thanks to Juraj's <a href="https://arduino.stackexchange.com/users/37622/juraj">comment</a> for the tip </sub></p>

<blockquote>
  <p><sub>this is not the current STM boards package. ST Microelectronic took over the development <a href="https://github.com/stm32duino/Arduino_Core_STM32" rel="nofollow noreferrer">github.com/stm32duino/Arduino_Core_STM32</a> <a href="http://stm32duino.com/viewtopic.php?f=16&amp;t=4671" rel="nofollow noreferrer">stm32duino.com/viewtopic.php?f=16&amp;t=4671</a></sub></p>
</blockquote>

<h3>Step by Step</h3>

<p>The process is as follows (screenshots from OS X, but on Windows/Linux the process is the same):</p>

<p>Adding the URL in the preferences dialog (one URL per line)</p>

<blockquote>
  <p><a href="https://i.stack.imgur.com/huCZ3.png" rel="nofollow noreferrer" title="STM32 - Additional Boards Manager URLs"><img src="https://i.stack.imgur.com/huCZ3.png" alt="STM32 - Additional Boards Manager URLs" title="STM32 - Additional Boards Manager URLs"></a></p>
</blockquote>

<p>Boards Manager menu</p>

<blockquote>
  <p><a href="https://i.stack.imgur.com/9TNTA.png" rel="nofollow noreferrer" title="STM32- Boards Manager menu"><img src="https://i.stack.imgur.com/9TNTA.png" alt="STM32- Boards Manager menu" title="STM32- Boards Manager menu"></a></p>
</blockquote>

<p>Boards Manager dialog</p>

<blockquote>
  <p><a href="https://i.stack.imgur.com/sRmCN.png" rel="nofollow noreferrer" title="STM32 - Boards Manager"><img src="https://i.stack.imgur.com/sRmCN.png" alt="STM32 - Boards Manager" title="STM32 - Boards Manager"></a></p>
</blockquote>

<p>Boards Manager dialog - STM32 Core Boards installed</p>

<blockquote>
  <p><a href="https://i.stack.imgur.com/B7AwL.png" rel="nofollow noreferrer" title="STM32 - STM32 Cores selected"><img src="https://i.stack.imgur.com/B7AwL.png" alt="STM32 - STM32 Cores selected" title="STM32 - STM32 Cores selected"></a></p>
</blockquote>

<p>Boards Manager dialog - Boards installing</p>

<blockquote>
  <p><a href="https://i.stack.imgur.com/4jVJN.png" rel="nofollow noreferrer" title="STM32 - STM32 Cores installing"><img src="https://i.stack.imgur.com/4jVJN.png" alt="STM32 - STM32 Cores installing" title="STM32 - STM32 Cores installing"></a></p>
</blockquote>

<p>Boards Manager dialog - Boards installed</p>

<blockquote>
  <p><a href="https://i.stack.imgur.com/8h3np.png" rel="nofollow noreferrer" title="STM32 - STM32 Cores installed"><img src="https://i.stack.imgur.com/8h3np.png" alt="STM32 - STM32 Cores installed" title="STM32 - STM32 Cores installed"></a></p>
</blockquote>

<p>Here are the newly installed boards in the menu</p>

<blockquote>
  <p><a href="https://i.stack.imgur.com/vDwJu.png" rel="nofollow noreferrer" title="STM32 - Boards Manager added boards"><img src="https://i.stack.imgur.com/vDwJu.png" alt="STM32 - Boards Manager added boards" title="STM32 - Boards Manager added boards"></a></p>
</blockquote>

<p>As a comparison, these are the boards installed using the deprecated manual process described below:</p>

<blockquote>
  <p><a href="https://i.stack.imgur.com/HbFWW.png" rel="nofollow noreferrer" title="STM32 - Manually added boards"><img src="https://i.stack.imgur.com/HbFWW.png" alt="STM32 - Manually added boards" title="STM32 - Manually added boards"></a></p>
</blockquote>

<hr>

<h3>Deprecated method</h3>

<p><strike>
From <a href="https://wiki.stm32duino.com/index.php?title=Installation" rel="nofollow noreferrer">STM32 - installation</a></p>

<blockquote>
  <h3>Prerequisites</h3>
  
  <p>If your STM32 board does not have a bootloader yet and you want to be able to program it over USB, you'll have to upload
  the STM32duino bootloader to flash memory. See <a href="https://wiki.stm32duino.com/index.php?title=Burning_the_bootloader" rel="nofollow noreferrer">Burning the
  bootloader</a>.</p>
  
  <h3>Arduino and Arduino_STM32</h3>
  
  <ul>
  <li>Download and install the official <a href="http://arduino.cc/en/Main/Software" rel="nofollow noreferrer">Arduino IDE</a>. Versions 1.6.10 till 1.8.5 have been reported to work fine.
  
  <ul>
  <li>If you already have the Arduino IDE installed and it's a newer version, see the <a href="https://wiki.stm32duino.com/index.php?title=Boards_Manager_package" rel="nofollow noreferrer">Boards Manager package</a> method of installation.
  
  <ul>
  <li>Note: this method is deprecated and not supported in forum!</li>
  </ul></li>
  </ul></li>
  <li>Run Arduino IDE, and open the menu <strong>Tools -> Boards -> Boards Manager</strong>. Install the "Arduino SAM Boards" package (includes the
  Arduino Due) from the list of available boards. This installs compiler
  support for ARM Cortex-M3.</li>
  <li>Download a zip file containing the Arduino STM32 files from <a href="https://github.com/rogerclarkmelbourne/Arduino_STM32/archive/master.zip" rel="nofollow noreferrer">here</a>.</li>
  <li>Unzip the content of the zip file, and place the Arduino_STM32 folder to <code>[Arduino sketches folder]/[hardware]/[Arduino_STM32]</code>.
  Create the '<code>hardware</code>' folder if it does not exist yet.</li>
  </ul>
  
  <p>Example how the path should look like:
  <code>C:\Users\&lt;user&gt;\Documents\Arduino\hardware\Arduino_STM32</code></p>
</blockquote>

<hr>

<p>As an aside, for anyone using OS X, from <a href="https://wiki.stm32duino.com/index.php?title=Installation_on_OSX" rel="nofollow noreferrer">Installation on OSX</a></p>

<blockquote>
  <h3>Installation on OSX</h3>
  
  <ul>
  <li><code>Arduino_STM32</code> folder must be placed inside
  <code>~/Documents/Arduino/hardware</code> (note. if the hardware folder doesn't
  exist you will need to create it) </li>
  <li>So you should get <code>~/Documents/Arduino/hardware/Arduino_STM32</code></li>
  <li>Note.  DFU util binaries
  have now been added to the repo, in <code>tools/macosx/dfu-util</code>, so there is
  no longer a need to install Homebrew to then install <code>dfu-util</code>, or to
  compile <code>dfu-util</code> from source.
  </strike></li>
  </ul>
</blockquote>
