Jealousy is Insecurity - It could be that this lack of security is very well founded--that the partner is about to run off with 'the other woman'. Is it okay to be bothered by that? Of course it is.
In some cases, the insecurity is not founded on realistic dangers to the relationship. If that is really the case, then you (the jealous partner) may wish to consider where your insecurities are coming from. Solving those sorts of insecurities isn't easy, but until you do you'll continue to face those feelings.
Since we tend to become more secure in relationships as they become more stable with time, you may find that time is your ally in dealing with jealousy.
There are things you can do to mitigate or help existing insecurities. There are things you can do to avoid insecurities before they happen. The latter things tend to be cheaper and more effective.
If a partner does something which triggers the insecurity of the other, that insecurity can feel like a breach of trust, and that is very difficult to heal. In particular, these feelings often come up in relationships. So, avoiding breaking trust in the first place is pretty important.
You can avoid the loss of trust to some effect through preparation. If you set your limits with the goal of avoiding insecurity, if you make avoiding the creation of insecurity a goal, then mistakes or misunderstandings can be dealt with in an environment that is still sending the right messages... and trust may not be as damaged.
Consider your needs and commitments, too, before setting limits. So, be clear with all your partner what you are willing to commit to and what you aren't, and if you stick by those commitments, you'll build a sense of trust with your partner.
One of the biggest mistakes you can make is to try and hide it (in the long term, that doesn't mean you have to display it at the moment you feel it.) Jealousy is usually a signal of something needing fixing, and ignoring that usually only makes things worse.
Instead, ask yourself "What do I feel insecure about?" Do I feel unattractive or uninteresting myself? Do I doubt the other persons love for me? Their physical attraction? Do I doubt that I can have the type of relationship I want?
Once you've figured out what the core discomfort is, then it's appropriate to ask yourself if your fears are well-founded. If it is, perhaps talking to your partner about what you are afraid of and why you believe your fears are well-founded makes sense.
A different case is where you are pretty sure that your fears are unwarranted, but you just feel insecure (often about yourself more than the relationship) anyhow. That is a more difficult case, and it's hard to know precisely what to do about it. Professional counseling can be a good way of dealing with unfounded internal fears. Talking through your jealousy and the trigger points for your feelings can help you gain insight into your own inner workings. Someone objective, someone neutral, that can lead you through points of discovery - gaining insight.
Here is a good article on how to defeat jealousy. Wishing you success. Jealousy: How to Kill the Green-eyed Monster