HYIPs (High Yield Investment Programs) have grown immensely popular and can now be found all over the Internet. While it's possible that some HYIPs make money, many of them are short-lived scams that typically promise ridiculous, unsustainable returns as high as 50% a month or more. If you have done any traditional investing (stocks, bonds, options, etc.), then you know that even 50% a year is extremely difficult to attain. So right away, a knowledgeable investor should be very suspicious of an opportunity that offers such an unbelievable rate of return.
It is widely believed that most HYIPs are complicated scams based on Ponzi schemes, where the returns provided to existing investors are paid out by the investments made by new investors who join the program. This type of scheme does not create any real value or wealth, and therefore, it eventually falls apart, usually within a year or two. If you do not pull out your funds before this breakdown occurs, you could stand to lose all your money.
Because it is so hard to discern which HYIPs are for real, and since the majority of them are most likely scams, my advice is to avoid them altogether. I have never invested in one, and never plan to.