The real driver of a bubble is the idea of a "get rich quick" investment mechanism that pulls in rewards out of proportion to risk, which inflates the capital within that investment, which inflates the bubble. Are we there yet? I don't see it.<p>When I think "bubble", I think of two - the old dotcom bubble, and the housing bubble. The housing bubble was driven by those nasty CDOs, which produced abnormal rates of return on AA/AAA securities, while leaving their value vulnerable to problems with the sub-prime market. Money pours into the housing market thanks to excessive returns, which both drives up housing prices and drives demand for more mortgages, which reduces proper risk management... a vicious cycle, til the subprimes start defaulting and the prices drop and then the bubble pops.<p>The old dotcom bubble was driven by IPO money from unsophisticated investors, so there was a massive push to get companies public as soon as possible, well before they had solid business models. This drew institutional money into the venture capital market, and again, things got all out of wack, and capital supply was driving startup demand, and weak businesses were getting funding they didn't deserve.<p>So the real marker for a bubble now, imho, isn't whether prices for early stage startups are going up, but rather whether a lot of bad startups are getting funded. Is that really happening? An increase in price suggests otherwise. Supply and demand, people. Sure, demand may be up, but either that increases price, or the market responds by increasing supply. And since the number of quality startups is basically fixed, supply increase means introducing bad startups.<p>And this doesn't even bring in other factors, like pg's marvelous observations about how startups need less Series A money these days generally, and the rise of super-angels, angel syndicates, and online tools like AngelList and Gust.<p>tl;dr Demand != bubble