user848: Is it possible to connect an Arduino to an Android Phone and use it for input/output?
Are connections possible over USB? Or do I need to have a wifi or Bluetooth shield?

user8: The Arduino ADK is meant for that. It combines an Arduino Mega with a USB host chip and port. If you don't want to buy a full Arduino, you can buy a USB host shield. Excellent tutorial for it here.
What those do is convert high speed USB signals to SPI, which is readable by a microcontroller. Android has libraries known as the Accessory Development Protocol, which can bridge communication from a phone to the USB and then to a microcontroller.

user220: I already had this answer typed up on another question so I thought I'd post it here to help you. I've replaced "tablet" with "phone" for easy reading.

You never mentioned your phone model... or Arduino model
There are some phones designed to work with USB flash drives and
  printers. This is called OTG (on the go.
You need:

A phone supporting OTG
An OTG adapter that fits into your phone's USB slot and is compatible
The Arduino Uno Communicator App
Arduino Uno (or clone) [Note: It says that it works with Atmega16U2 or Atmega8U2 programmed as a USB-to-serial converter so I would assume
  that that would cover a few boards more than the Uno.)

Another alternitive is to look into
  Bluetooth
  (Note: I just picked a random link but there are hundreds of similar
  BT adapters)

From my answer on a similar question

user8187: You can connect Android and Arduino with usb and use the easy tool App inventor 2 for android 
Did you try this new application : https://play.google.com/store/apps/details?id=bp.usbbridge.appinvDemo and full version : https://play.google.com/store/apps/details?id=bp.usbbridge.appinv
Find Videos about Explications : USB OTG Bridge Serial UART for APP Inventor 2 and ARDUBLOCK on youtube

user813: You bet it's possible. I used a pretty cheap bluetooth module that I got from Amazon, and wrote an app using the information on the official Android Developers page to connect to the Arduino. There are many guides online telling you how to hook up a module to the pins of the duino. 
You can probably get a cheaper module if you look on eBay and don't care how long it takes to arrive, and I think there's at least one app on the Play store for controlling an Arduino if you can make it work for your purpose.
USB connections are probably possible, though I haven't done any of that.

848: <p>Is it possible to connect an Arduino to an Android Phone and use it for input/output?
Are connections possible over USB? Or do I need to have a wifi or Bluetooth shield?</p>
 8: <p>The <a href="http://arduino.cc/en/Main/ArduinoBoardADK" rel="nofollow">Arduino ADK</a> is meant for that. It combines an Arduino Mega with a USB host chip and port. If you don't want to buy a full Arduino, you can buy a <a href="https://www.sparkfun.com/products/9947" rel="nofollow">USB host shield</a>. Excellent tutorial for it <a href="http://labs.arduino.cc/ADK/Index" rel="nofollow">here</a>.</p>

<p>What those do is convert high speed USB signals to SPI, which is readable by a microcontroller. Android has libraries known as the Accessory Development Protocol, which can bridge communication from a phone to the USB and then to a microcontroller.</p>
 220: <p>I already had this answer typed up on another question so I thought I'd post it here to help you. I've replaced "tablet" with "phone" for easy reading.</p>

<blockquote>
  <h2>You never mentioned your phone model... or Arduino model</h2>
  
  <p>There are some phones designed to work with USB flash drives and
  printers. This is called OTG (on the go.</p>
  
  <p><strong>You need:</strong></p>
  
  <ul>
  <li>A phone supporting OTG</li>
  <li>An OTG adapter that fits into your phone's USB slot and is compatible</li>
  <li><a href="https://play.google.com/store/apps/details?id=com.primavera.arduino.listener" rel="nofollow noreferrer">The Arduino Uno Communicator App</a></li>
  <li>Arduino Uno (or clone) [Note: It says that it works with Atmega16U2 or Atmega8U2 programmed as a USB-to-serial converter so I would assume
  that that would cover a few boards more than the Uno.)</li>
  </ul>
  
  <p>Another alternitive is to look into
  <a href="https://www.google.com/search?q=arduino+bluetooth&amp;qscrl=1&amp;tbm=isch&amp;imgil=WyPC4O05Xn4rhM%253A%253Bhttps%253A%252F%252Fencrypted-tbn2.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9GcSfTtWqIuN9YYdd90OO_tlxsvLUYhV_pl7NfaMhHDzSRq9N6W9rNA%253B1200%253B1200%253B1soQc4CzMzh0MM%253Bhttp%25253A%25252F%25252Fm.dhgate.com%25252Fproduct%25252Fjy-mcu-arduino-bluetooth-wireless-serial%25252F151048482.html&amp;source=iu&amp;usg=__KyMLS4aeubxJnDaMvprgRgADxAs%3D&amp;sa=X&amp;ei=1BUCU635IYmEygGO1oHwCQ&amp;ved=0CGkQ9QEwBg&amp;biw=1280&amp;bih=899" rel="nofollow noreferrer">Bluetooth</a>
  (Note: I just picked a random link but there are hundreds of similar
  BT adapters)</p>
</blockquote>

<p><em><a href="https://arduino.stackexchange.com/a/272/220">From my answer on a similar question</a></em></p>
 8187: <p>You can connect Android and Arduino with usb and use the easy tool App inventor 2 for android </p>

<p>Did you try this new application : <a href="https://play.google.com/store/apps/details?id=bp.usbbridge.appinvDemo" rel="nofollow">https://play.google.com/store/apps/details?id=bp.usbbridge.appinvDemo</a> and full version : <a href="https://play.google.com/store/apps/details?id=bp.usbbridge.appinv" rel="nofollow">https://play.google.com/store/apps/details?id=bp.usbbridge.appinv</a></p>

<p>Find Videos about Explications : USB OTG Bridge Serial UART for APP Inventor 2 and ARDUBLOCK on youtube</p>
 813: <p>You bet it's possible. I used a pretty cheap bluetooth module that I got from <a href="http://rads.stackoverflow.com/amzn/click/B0093XAV4U" rel="nofollow">Amazon</a>, and wrote an app using the information on the official <a href="http://developer.android.com/guide/topics/connectivity/bluetooth.html" rel="nofollow">Android Developers page</a> to connect to the Arduino. There are many guides online telling you how to hook up a module to the pins of the duino. </p>

<p>You can probably get a cheaper module if you look on eBay and don't care how long it takes to arrive, and I think there's at least one app on the Play store for controlling an Arduino if you can make it work for your purpose.</p>

<p>USB connections are probably possible, though I haven't done any of that.</p>
