I've been trying to confirm this but thus far in vain. When I use the internal pull up resistor, and I connect to ground, specifically the Ground pin (not some external ground), do I also need a resistor in to ensure no damage to my Arduino board? I think I do not need a resistor before GND, but I am just a tad apprehensive since I don't want to damage my board with full pin output into GND.No, when the pin is configured as an input pin and then the internal pull-up option is enable, it is prefectly safe to ground the input pin. The risk with direct grounding of a I/O pin is that if it happens to be set as a output pin and set HIGH (either by user error or software bug, same thing really) then there will be pin damage as that is a direct short circuit from Vcc to ground through the pin.Also, it might be the time of night though...will I damage my board if I enable say 5 pins' internal pull-up resistor all to the GND pin? I've just gotten it in my head for some weird reason, that if I push all 5 buttons, the GND will receive 25 volts? But that doesn't make sense either. I am very sure I have just over thinked this :~ :~ :~ :~ :~ :~ :~ :~No, you need more sleep. Doesn't matter how many INPUT pins you short to ground there can be no damage. Only when pins are set to ouput mode do you need to concern yourself and understand about current flow and current limits.