user1411: I have arduino pro mini 5V, 16M ATMega328 and CP2102 USB to TTL. I've wired them like this:

When I try to upload a sketch to the arduino board I'm getting the following error:
avrdude: stk500_getsync(): not in sync: resp=0x00

The board type and the serial port are correct. The sketch also compiles successfully and I've installed the CP2102 drivers. I've noticed that even if I unplug all wires from the arduino board(and just leave the cp2102 connected to the PC), I'm getting the same error again. I'm using Windows 7.
Anyone else encountered the same issue?

user10794: As some of the other answers have said, you may not have soldered on the header pins. You can't just push them into place and hope for the best. See my Pro Mini:

My soldering is reasonably neat, however you can see the solder come through to the other side.
Also see:

Some dude named avr won't let me upload my program, aka “avrdude: stk500_getsync(): not in sync: resp=0x00”
Have I bricked my Arduino Uno? .

A lot of the issues in those threads apply to any Arduino.

user48525: To solve my sync issue I added a connection from USB serial adapter RST to ProMini DTR pin. I'm using Linux Mint 18 OS with Arduino IDE 1.8.5 -
My other Arduino clones (UNO_r3 and Nano) uses the CH340G chip so I knew I had working drivers. Sorry for the picture quality, my old camera and I don't know how to balance LED light bulbs. The pins from USB to ProMini are: (U)GND-(P)GND; (U)3V3-(P)no connection; (U)5V-(P)VCC; (U)TX-(P)TX; (U)RX-(P)RX; (U)RST-(P)DTR.  Hope this helps. 73 AB1OP_Ralph


user4729: I solved this problem by connecting DTR signal from USB-converter to the pin which says GRN on the board (the one aside TXD). It was not obvious to find, but seen on ebay some arduino-dedicated-usb-programmer which seemed to be connected this way - so I took the chance. I do not get this error anymore and uploading works every time!!!
Cheers!

user367: This error means that the program responsible for uploading (avrdude) can not communicate with your arduino. 
Did you reset the board just before programming? The reset pin of the arduino seems unconnected in your picture; so the board wouldn't be executing the bootloader when the serial cables expects it to. 
Try either to reset the board manually just before programming, or connect the rst pin.

user1217: It's hard to tell from your pictures, but are all the header pins soldered to the boards? If not, they need to be: there's no chance that the proper electrical connections will be made reliably if they're just inserted loosely into the holes.

1411: <p>I have arduino pro mini 5V, 16M ATMega328 and CP2102 USB to TTL. I've wired them like this:</p>

<p><img src="https://i.stack.imgur.com/Zlrea.png" alt="enter image description here"></p>

<p>When I try to upload a sketch to the arduino board I'm getting the following error:</p>

<pre><code>avrdude: stk500_getsync(): not in sync: resp=0x00
</code></pre>

<p>The board type and the serial port are correct. The sketch also compiles successfully and I've installed the CP2102 drivers. I've noticed that even if I unplug all wires from the arduino board(and just leave the cp2102 connected to the PC), I'm getting the same error again. I'm using Windows 7.</p>

<p>Anyone else encountered the same issue?</p>
 10794: <p>As some of the other answers have said, you may not have soldered on the header pins. You can't just push them into place and hope for the best. See my Pro Mini:</p>

<p><img src="https://i.stack.imgur.com/zgG73.jpg" alt="Arduino Pro Mini"></p>

<p>My soldering is reasonably neat, however you can see the solder come through to the other side.</p>

<p>Also see:</p>

<ul>
<li><a href="https://arduino.stackexchange.com/questions/17/some-dude-named-avr-wont-let-me-upload-my-program-aka-avrdude-stk500-getsync">Some dude named avr won't let me upload my program, aka “avrdude: stk500_getsync(): not in sync: resp=0x00”</a></li>
<li><a href="https://arduino.stackexchange.com/questions/13292/have-i-bricked-my-arduino-uno">Have I bricked my Arduino Uno?</a> .</li>
</ul>

<p>A lot of the issues in those threads apply to any Arduino.</p>
 48525: <p>To solve my sync issue I added a connection from USB serial adapter RST to ProMini DTR pin. I'm using Linux Mint 18 OS with Arduino IDE 1.8.5 -
My other Arduino clones (UNO_r3 and Nano) uses the CH340G chip so I knew I had working drivers. Sorry for the picture quality, my old camera and I don't know how to balance LED light bulbs. The pins from USB to ProMini are: (U)GND-(P)GND; (U)3V3-(P)no connection; (U)5V-(P)VCC; (U)TX-(P)TX; (U)RX-(P)RX; (U)RST-(P)DTR.  Hope this helps. 73 AB1OP_Ralph</p>

<p><a href="https://i.stack.imgur.com/fqjwG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fqjwG.png" alt="CH340G USB serial adapter to Arduino ProMini"></a></p>
 4729: <p>I solved this problem by connecting <strong>DTR</strong> signal from USB-converter to the pin which says <strong>GRN</strong> on the board (the one aside TXD). It was not obvious to find, but seen on ebay some arduino-dedicated-usb-programmer which seemed to be connected this way - so I took the chance. I do not get this error anymore and uploading works every time!!!</p>

<p>Cheers!</p>
 367: <p>This error means that the program responsible for uploading (avrdude) can not communicate with your arduino. </p>

<p>Did you reset the board just before programming? The reset pin of the arduino seems unconnected in your picture; so the board wouldn't be executing the bootloader when the serial cables expects it to. </p>

<p>Try either to reset the board manually just before programming, or connect the rst pin.</p>
 1217: <p>It's hard to tell from your pictures, but are all the header pins soldered to the boards? If not, they need to be: there's no chance that the proper electrical connections will be made reliably if they're just inserted loosely into the holes.</p>
