In a perfect relationship, you wouldn't keep any secrets from your sweetie. And you'd never have to worry if he were keeping anything from you, because his life, too, would be an open book.

But we live in the real world, where even the healthiest couples sometimes hide things from each other. To most of us, the secret to end all secrets (and many relationships) is an affair -- and no one will quibble with the devastating consequences of infidelity. Yet even "small" deceptions can rock a relationship, and it can be hard to draw the line between what's harmless and what's not.

So how to tell what can stay safely tucked away and what calls for a confession? We asked dating experts to outline the rules for secret keeping (and sharing).

If you want a relationship grounded in mutual trust (and who doesn't?), certain issues require full disclosure. "If something has a chance of impacting your partner's future or his life with you, then he has a right to know about it," says Mira Kirshenbaum, a relationship expert and author of "Is He Mr. Right?" This includes anything from the past that has reverberations in the present (lingering debts, a chronic medical condition, past emotional abuse), and anything in the present that could affect the future (a health scare, a potential downsizing at work).

As many couples find out too late, when you keep a secret that profoundly affects your family, you face a double whammy when the secret eventually -- or inevitably -- implodes: After the first shock wave from the hidden truth rips through the family, you're hit with the secondary tremors that come from the resulting feelings of betrayal and distrust. "My husband was running a retail website, and he assured me it was doing well," recalls Karen, a 39-year-old mom of two. "I thought I could stay home with my children and not have to worry about making money myself." A year ago, however, Karen discovered that her husband's business was in the hole for more than $1 million, and he had taken out a line of credit against their house. The truth came out only when he announced that he would have to take an additional job. "I was furious!" Karen says. A year later, her family has begun recovering from the financial blow, but Karen is still dealing with her sense of hurt, anger, and betrayal.

When confronted with such a big, sudden revelation, it's natural to think, "Why didn't you tell me? I'm the one person on earth who is supposed to understand!" But the reasons men, in particular, keep secrets like these are multilayered, say the experts. They may, like Karen's husband, want to keep up an appearance of being strong and in control -- out of fear that you'll stop loving them if you see them in a different, vulnerable light. They may be loath to deal with the turbulence they know their secret will unleash. And yes, some men may just hope that silence will help them avoid a fight. "Men are wary of women's emotional reactivity and usually shy away from conflicts," says Redbook Love Network expert Scott Haltzman, M.D., author of "The Secrets of Happily Married Men." "They'll do anything to avoid making a woman furious, though they usually end up making things worse."

Even when your secret is something that seems minor to you, you must be open with your partner if it's related to a topic he's expressed curiosity or a strong opinion about. "As soon as you get serious with someone, sit down and ask him, 'Is there anything you want to know about me that I haven't told you yet?'" advises Redbook Love Network expert Harville Hendrix, Ph.D., author of "Getting the Love Your Want." It's a clear-the-air strategy even long-term couples can use. And as counterintuitive as this may sound, the more likely your partner is to freak out about your secret, the more important it is that you confess, Hendrix adds, because if the truth about a sensitive subject comes out later, the rupture could be even more serious. Not only does your partner need all of this information to be able to make informed decisions about your life together, he also needs to know that you trust him enough to share it. "If you don't tell your partner about something, then you're really saying he's not wise enough, generous enough, or loving enough to handle it," says Kirshenbaum. "And that can be deeply wounding."

If you're honest with yourself, then it's not too hard to figure out which secrets fall into the "Red Light! Stop and Spill Everything" category. But what about the ones -- especially those transgressions from your distant past -- that belong in the murkier "Slow Down and Think Before You Spill" zone? On the one hand, if some embarrassing episode from your history is long past and has no impact on your life today, you can consider keeping your lips sealed.

Here's another important factor when weighing whether or not to tell. Murphy's Law of love says that the less you want a secret to be revealed, the more likely it is to suddenly spring up in your relationship like a jack-in-the-box, shouting, "Gotcha!" So consider this: Are there any pictures floating around on the Internet that might give you away? Any friends who have a habit of telling your somewhat sordid old stories after a couple of mojitos? How about an ex-lover who could potentially reappear? If there's any way your partner might accidentally learn your secret, it's better that you do the telling first.

Coming clean about your romantic past is a worthy goal, but Kirshenbaum warns that you must tread carefully. "You don't need to give the exact number of previous lovers or endless details about what you did with them," she says. "There's absolutely no point in filling your partner's head with mental images that can't be deleted."

Sure, as a rule, honesty is the best policy. But anyone who's been in a long-term relationship knows that there is definitely some wiggle room when it comes to keeping a few personal tidbits to yourself, provided they are harmless and don't involve any outright lies. Maybe you earned a bonus and spent it all on a new leather jacket without telling him; or he listens to Howard Stern on the radio on his way to work and doesn't tell you. "Even when we're married, we still have private selves," says Kirshenbaum. "It's healthy to have a sense that this is my personal business and no one else's." In fact, keeping an innocent little part of yourself off-limits can add some spark to your relationship. If he knew every last detail about your life, what fun revelations could there possibly be in the future?

Some couples find that hanging a veil of secrecy over certain aspects of their lives helps make their marriages run much more smoothly. "For the first decade of our marriage, my husband and I fought over every purchase I made, from a bag of cleaning supplies to a new coat," says Laurel, 36. "Finally, I had this breakthrough: if he doesn't see a bill, he won't think about it! So now I use cash when I shop. If I want to buy a pair of shoes, and I can afford them, he doesn't have to know how much they cost. Believe me, everyone in the house is a lot happier now."

Just make sure you and your guy are on the same page. If you're both willing to trade complete candor for a bit of ignorance-is-bliss harmony, then there's no harm, and plenty of potential gain. But, Haltzman points out, this deal only works when it goes both ways. Don't harass him about the cost of his new camera lens if you don't want him to ask you the price of that new handbag -- and when it doesn't involve an area you are working on together, like a budget. "If your partner has expressed a desire to work cooperatively on something and you're still keeping information from him, then you've crossed that line," Haltzman says.

One clue that your secret is a healthy one: The evidence is usually hidden in plain sight. If your partner really wanted to know how much those shoes cost, he could notice the fancy label on the insole; if you really wanted to know what offensive radio show he was listening to, you could click on his preprogrammed radio stations next time you're in the car.

Some deceptions are just plain burdensome, often creating more anxiety and distress than coming clean ever would. "I agonized about my secret for the first several months I was dating my now husband," says Cathy, 41. "I'm 10 years older than he is, and I was sure he would dump me if he found out." Her day of reckoning came when they joined Paul's mother for dinner at a Chinese restaurant. "She looked on the place mat and said, 'I was born in the year of the monkey. Which year are you?'" Cathy steeled herself and told the truth, revealing her age. "I was sure my boyfriend would break up with me, but he just laughed and said he didn't think I was too old for him. He was worried that I would think he was too young for me!" Proof that the truth has a way of coming out -- and when you've got a partner you trust, it doesn't have to be so awful after all.

Here's the smart and sensitive way to tell your guy what you've been hiding:

Make an appointment. Don't just spring it on him. Say, "I have something important to discuss with you. Can we find an hour tonight to talk?"

Pick the right spot. Avoid spilling in a crowded public place, like a restaurant, or your bedroom, which should be reserved for positive experiences. Find a safe, neutral spot, like the den or a park.

Be prepared to apologize. "The big mistake is trying to make it seem as if the revelation is no big deal," says marriage counselor Mira Kirshenbaum. "You need to say, 'I'm sorry, there's something I should have told you a long time ago, but I was ashamed to tell you. I hope you'll forgive me.'"

Enlist a third party. A major reveal works best with a referee. Recruit a friend you both trust -- or in the case of the biggest bombshells, such as an affair, a marriage counselor.