Last week was the 30th birthday of the Vanguard First Index Investment Trust, now known as the Vanguard 500 Index Fund.

It wasn't the first index fund ever, but it was the first one marketed to small-time investors. And it may not have started a movement all by itself, but it was certainly there at the start of the index revolution.

In 30 years, indexing - the simple idea of buying a basket of stocks designed to capture the performance of a broad market - has grown up and gotten complicated. Today more than 1,000 mutual and exchange-traded funds call themselves index funds, and they track dozens of indexes.

The concept behind index investing is elegant and has won many converts. Indexers can invest in broadly diversified portfolios without spending much money. They can grab most of the market's long-term winning returns without having to spend time and money picking their own stocks or paying expensive fund managers to do it for them.

Most of the time, index funds perform better than most actively managed portfolios. And because they tend to have lower turnover than actively managed funds, their tax burdens are often lower.

If you buy into all that (and many very knowledgeable financial advisers do), then index funds make a lot of sense for the core of a long-term investment portfolio.

Choose your index, buy the correlating fund, and spend the rest of your free time playing tennis or cooking or hanging out with your kids: Your investment work is largely done.

But what 30-year-old doesn't have issues? The more complicated index investing gets, the more decisions it entails. Here are some of the questions facing indexers now.

- Is active indexing a contradiction in terms? The theory behind index investing has always held that the value of buying an index fund was in not having to figure out which companies would do well and which wouldn't, or which stock-picking strategy was superior or timely.

But the newest of these funds are based on indexes with a selective component. Many, such as those issued by relative newcomer WisdomTree Investments, are based on indexes of stocks weighted by their dividend yields. The market is only moments away from creating indexes of companies with high earnings growth rates or cash balances or the like.

At some point an index defined by investment criteria isn't an index anymore; it's a bet that value will do better than growth or vice versa. And if you're making those bets, you're not really indexing in the purest sense of the word. You're becoming a quant, or quantitative investor, and that means you've put your money behind a particular stock-picking strategy. That may not be bad, but it's different.

- Should you time the market or not? Index funds are part of a grander scheme called modern portfolio theory: the idea that you should allocate your portfolio among assets like stocks and bonds, use cheap index funds to be in those positions, and then hold tight, selling funds only when you need to correct imbalances in the mix. But many investors have also noted that indexes are the perfect vehicle for market timers.

Think stocks are going to go up this fall? Just load up on shares of a stock index fund, and sell it when you think the tide has turned. There's a lot of academic research behind the statement that market timing is nigh impossible, but in recent years, there are also some market timers who have done very well.

- How narrowly can you slice and dice the market? It's one thing to buy a total market index fund, or even a European stock index fund or a small-cap index fund. But now there are dozens of ETFs that define indexes as narrowly as the companies in a single economic sector.

There's no evidence that the big, broad powers of indexing pay off in those circumstances. If you're going to spend all of your time choosing sectors or types of companies, perhaps you should go one step further and choose the best stocks in those categories.

- Fees really matter. The irony is that indexing was initially heralded as a cheap way to play the market, but some of the new funds are pretty costly. The more work that goes into creating an index, the more the fund managers have to charge. Even plain vanilla S&P 500 index funds can have a wide diversity of fees and expenses, and the higher the fees, the lower the returns.

- Index investing doesn't always work as promised. Over the last three years, more than 400 mostly actively managed funds have done better than the bargain-priced Vanguard 500 index fund (in its own large-cap growth/value blend category), according to research firm Morningstar.

It may be that the market has trended sideways for much of 2006 in a pattern that some people call a stock picker's market. There's also a fair amount of research demonstrating that some indexes, such as those tracking emerging-market or small-cap companies, don't do as well against actively managed competitors as the big, broad market indexes.

Questions aside, if you're committed to the concept that indexes beat active management over time, then you shouldn't switch to stock-picking when the market is challenging. The whole concept of index investing is based on that premise.