By Jane Edmondson Mid-cap companies should be a key component of a well-diversified portfolio, in my opinion. While mid-cap stocks are positively correlated with small- and large-cap stocks, they still offer diversification benefits for a multi-asset portfolio. Furthermore, mid-cap stocks as an asset class have demonstrated better historical returns than small- and large-cap equities. Investing in mid-cap stocks gives you exposure to mid-sized companies ranging in size between $1 and $10 billion in market capitalization. Some of today's leading brand names such as from Alaska Airlines (ALK), Autozone (AZO), Domino's Pizza (DPZ), Hershey's (HSY) and Under Armour (UA) are all mid-cap companies. These middle-of-the-pack players often come with less risk than small caps and comparable risk to large caps.