Oh he has taken drugs, including EPO and testosterone, and blood doping. Later EPO was microdosing
Seriously, the amount of evidence is overwhelming. I am going to paste some links, and before anyone else argues he was clean, please read them.
As scarfy said, the whole reason he gave up is so he can keep up the charade. If he fought, it all would have come out.
Index of all allegations, including the Simeone affair:
http://www.cyclingnews.com/features/index-of-lance-armstrong-doping-allegations-over-the-years
Lance sued Simeoni and had some despicable treatment of him in the peloton for "daring" to out his doping doctor Ferrari. Of course, later on Ferrari would get a life ban for doping his riders. That is what you get for breaking the omertà - a bully like Armstrong ruining your pro career.
Just read a lot links from above, starting from his testosterone positive in 1999 that he got a backdated TUE for all the way to his controversy filled comeback, including stopping his third party auditing of his blood before it even began, to his dodgy (well, impossible blood values) in the TdF 2009, to his evading of a regular doping control for a few hours.
Of course, normally he knew when the "surprise" dope controls would occur:
http://www.cyclingnews.com/news/report-armstrong-warned-before-all-doping-controls
Now there is so much smoke, around a raging fire, including now a mass of ex team mates giving evidence that he doped. To my mind you have to be seriously naive or have your head in the sand that he didn't dope.
The thing though, is his treatment of people whom has called him out - he has tried to ruin their lives, including Greg LeMond and the Andreaus.