Short answer: As long as the portion isn't dominant in your portfolio, and you re-evaluate it at least twice a year and rebalance, its all good.<p>Longer answer, looking at Googles rates of return on capital is like watching a very, very, fast train driving along straight track. Google's nominal operating mode burns a lot of money, that burn rate leaves them vulnerable to down turns. What the most likely scenario is not that they get disrupted so much as others meet them at parity but run at an operationally much more efficient way. Say that M$/Bing inexorably creeps up into a neck and neck race with Google in terms of search reach, and the combination pushes down advertising rates. The spending brakes go on at the 'plex, the company changes, the bean counters rise in power beyond reasonable measure, and the company 'flips'. Out go the creative, inventive, people and in come the 'lifers' who know how to make their job look important without actually doing anything. The mechansim which makes this happen is really hard to avoid since from the board room things look fine, even after they aren't fine. And there is a fine balance of disbelief that is held between the employees and their condition that, once disrupted, cannot be re-established. Cisco, HP (Tandem), Sun, and Intel are all places where I've known folks who lived through the flip. Nothing is the same afterwards, if you can't re-invent yourself like Apple did, it gets a lot harder.<p>That being said, there is (as others point out) usually some time to cover for that.<p>I cannot stress enough however that financial planning is serious business and you should take your time with it. I believed them when they said if I started putting money in my 401K in 1986 that it would be worth multiple millions by now thanks to the 'miracle' of compound interest. They missed the part about the 'miracle' of the nations worst economic debacle deleting that value in a heartbeat. I don't believe them any more :-(