I was hesitant to buy the product because of many negative reviews that it didn't work, couldn't get it setup, etc.However, on couple sites, I saw it as one of the best smart thermostats at a decent price and decided to give it a try.Yes, it does require a C wire - if you are not sure what it means, it would help to check with your HVAC guy. Installation guide has also a method without using a C Wire (using Green wire for C Wire purposes), but then you loose the ability to control fan without Furnace/AC, if you want to run the fan only for example...Your furnace board needs to be able to support the C Wire connection regardless. The problem with the newer, more feature rich thermostats is that they need more power than just when the furnace/AC is on + couple AA batteries to keep the wifi connection and bigger/nicer displays, etc. C Wire provides that additional power from the furnace...I had a 4 wire (18-4) running from thermostat to furnace. Since I plan to stay in this house for the long haul and wanted to do it right, I bought the 18-5 thermostat cable (18 gauge each - 5 wires) and ran the new cable from thermostat to the furnace and removed the old cable. Connected to the furnace and to the thermostat all in less than 45 minutes. Turned on the power, followed the instructions to install the honeywell lyric app, connect wifi, setup the daily schedule from the app and furnace turned on within couple of minutes like nothing ever happened. Very happy with the setup and thermostat so far...Update 7 months later - still very happy with the product, buying a 2nd one for my 2nd furnace/ac...