By "using a debit card as a credit card", I'm assuming you mean paying for purchases. Is this dangerous? It can be. Is it a good idea? Almost never. I'll briefly explain.

First, debit cards lack the consumer protections of credit cards. Under the Fair Credit Billing Act, you have dispute rights an item you purchase with a credit card is defective, not as advertised, or is not delivered. While debit card issuers have policies that they claim offer similar protections, those policies do not have the force of law that credit card users enjoy. Moreover, since debit cards expose your bank account information to third parties, the potential consequences of fraud, vendor error, and the like are much greater than with a credit card.

Second, the price of mistakes when using a debit card tend to be greater. Most obviously, if I overdraw a bank account using debit charges, I can quickly and easily be liable for fees of $25 or more--even several fees at a time. If I am not mear to "maxing out" my credit card's limit (and I should never do this), then I will never incur such fees.

Finally, credit cards offer many other advantages that debit cards do not. Rewards, extended warranties, rental car insurance, and even price protection are just some "perks" offered via credit cards that debit cards do not match. And if I'm paying my balance in full each month, using a credit card gives me an interest free loan of the creditor's money to make my purchase. Celebrated columnist Liz Pulliam Weston just wrote a solid piece about these and other advantages credit cards provide.

One advantage of debit over credit reported by some users is that they find they are more disciplined about their spending when the money is coming directly from their bank account than they might be if they were using a credit card. But I would encourage such users to find other means of attaining such discipline, because the concrete advantages of using credit cards over debit cards for purchases are considerable indeed.