In language we link our thoughts to the physical world through names, symbols and pointing.  Similarly there need to be means for the digital and physical worlds to connect.  In this chapter we will look at the various ways in which this is managed from markers, such as the ubiquitous QR code, inserted into the physical world, to digital traces of physical phenomena, such as biometrics or image recognition.  Some of the marks in the environment are purely digital, such as NFC tags in clothes or credit cards, invisible to the human eye, but capable of tracking a device or person.  Others are more apparent and indeed may be aesthetically pleasing as well as functional.  Devices around us may be isolated, but increasingly talk to one another creating a web of sensing, or maybe surveillance.  Crucially once linked to digital representations they allow the physical world to affect the digital.

Keywords: physical-digital, tangible computing, tangible interaction, distributed computing, internet of things