A man once challenged the rabbinic sage, Hillel, to teach him the whole Torah while standing on one leg. Hillel responded that which is hateful to you, do not unto another. This is the whole Torah. The rest is commentary. Lots of people think that learning how to invest is as difficult as learning the whole Torah. I'm not a certified investment advisor, but here's my attempt to teach how to invest for retirement while standing on one leg. There are three simple rules. Tithe to yourself, follow the birthday rule, and invest in low cost index ETFs. The rest is commentary. Let me explain, number one. To make sure that you save enough, tithe to yourself. Save at least one tenth of your after tax income. You're probably screwing up if you're saving less than 5% of your income. Number two, to make sure that you expose enough of your savings to the stock market, follow the birthday rule. Take 120 minus your age, and invest at least that percentage in the stock market. So it you're 30, invest 90% of your savings in stock and invest the remainder of your savings in bonds. Do not sell stocks or bonds because you think the market is overvalued and going to crash. Number three, to make sure that you're diversified and have low fees, invest in stock and bond ETFs. Split your stock investment between Vanguard's total stock market ETF and Vanguard's total index international stock ETF. And split your bond investments between Vanguard's total bond market ETF and Vanguard's total international bond ETF. You're probably screwing up if your annual expense ratio is over half a percent and if your investments don't expose you to hundreds of stocks. So that's it. Tithe to yourself, follow the birthday rule, and invest in diversified ETF or mutual fund indices, with annual costs less than half a percent. If you follow these rules, you can be pretty confidant that you're saving enough, exposing your savings enough to the stock market, and cheaply diversifying enough. Usually, unusual circumstances might make it appropriate to deviate from these rules. But the vast majority of people who are currently deviating from these simple rules would do a lot better if they didn't. And if you feel the impulse to deviate, cheat just a little. There is a lot more to say including that you might want to overweight small caps stocks in your portfolio, or I think many young people can prudently invest in stock, more stock, than ordained by the birthday rule. But that will have to wait until I get better at balancing on one leg.