This replaces your existing chip on your T3661 waste tank when it reaches a point of saying it is full.  Of course it isn't REALLY full and we know people that are reporting that it can still take a lot more ink so you might be able to get away with up to 4 full cycles before you finally may want to replace the entire cartridge.  The entire cartridge STARTS at 10 bucks directly from Epson but then add on tax and shipping and the time it takes to come from Epson (they are out a LOT too) and this solution looks much easier.  If your Epson printer uses a T3661 waste tank, this will easily replace the chip on your cartridge now.  Put it back in your printer, it will show as NEW and you will have a whole new cycle to use it.  This works on the XP-15000 and also on the list below.