The USB to Serial adapter that comes with the Bittle would not work with my laptop. I spent several hours trying. Windows would simply list it as a "USB Serial" device under "Other devices" with a warning sign on it, but would never give it a COM port. I tried all of the suggested options: Installing FTDI drivers from the FTDI web site. Installing drivers from the Arduino install folder. Forcing Windows to use various drivers with this device. Etc. Some things would alter the appearance of the device in Device Manager, or change the behavior in some way, but nothing actually made it work. Then I remembered I had this in a box: https://lowpowerlab.com/shop/product/90 I plugged it in, and it worked immediately with the Bittle and Arduino on Windows. So for others having trouble with the FTDI or USB to Serial adapter that comes with Bittle, you may need to buy an alternate FTDI controller. I don't know for sure that https://lowpowerlab.com/shop/product/90 is the only option, as there are many available from other places like AdaFruit, SparkFun, and even Amazon. For instance, this one looks identical to mine:
and I guess it would work the same, but I can't say for sure, as I have not used it. (I noticed that https://www.adafruit.com/product/284 also includes the needed adapter, so that is a bonus.) Be aware that most, if not all, of the FTDI adapters come with the same connector as the Bittle's board, so you will need to adapt them together using something like this:
Notice how you can stick two things with holes together with the pins. You could probably use anything, including cutting of bits of paperclips, but I had these around and they worked perfectly. (I noticed that https://www.adafruit.com/product/284 also includes the needed adapter, so that is a bonus.) TL;DR: The included USB to Serial adapter may not work on your computer. A purchased FTDI controller like this https://lowpowerlab.com/shop/product/90 can work instead.