Investors love risk, as long as it's recognized and compensated for. A major kind of player in the global financial market attempts as its sole function to identify and label risk by levels, facilitating trading and easy comparisons. Ratings cover all the ground between “high quality” and “already in default.” If these credit rating agencies gives a high enough rating (above BBB-, by S&P terms), securities can be referred to as "Investment Grade.” U.S. Treasury securities are generally considered to have a rating above AAA (on the S&P) scale, with a correspondingly superlative rating for the others; for reasons discussed above, they are considered “risk-free.” Lower-rated bonds are relegated to "junk bond" or more optimistically, "high-yield bond” status. A person of significance for this topic is Michael Milken, who in the 1980s and1980s recognized an untapped potential in junk bonds and almost single handedly developed the market for them, making surreal amounts of profit (sometimes 100% return on investment), and later ending up in jail for illegal practices.