A job offer is usually welcome news—unless it comes while you're waiting to hear about a different job you really want with a different employer.
While many job-seekers might see this as a good problem to have, it's a tough spot to be in. And most job-seekers aren't sure how to navigate it well. After all, can you put the first company off, and if so, for how long? What should you say to the first company in the meantime? And can you take the offer but rescind your acceptance later if the other job comes through?
The first step here is to call the company that made you the job offer. We'll call it, "Company A." Explain that you're very interested in the job and would like some time to think it over, and ask when they need to hear back from you. Any reasonable employer will give you a bit of time—generally a few days to a week. Be aware, though, that many employers will balk at giving more time than that. Especially since a request for more than a week tends to signal that you're hoping for an offer from somewhere else in the interim and will make them question your interest level.
Next, contact the other company (Company B) immediately. Don't delay by even a day—time is crucial here. Explain to them that you have an offer from another company, that you need to give them an answer quickly, and that Company B is your first choice. If Company B is strongly interested in you, there's a good chance that they'll be willing to expedite things.
However, Company B might tell you that they can't speed up their timeline. If that happens, then you have a difficult decision to make: Are you willing to turn down Company A's offer without any guarantee that you'd get an offer from Company B in the future? Your answer probably depends on your financial situation and how confident you feel about other prospects coming along.
But what you shouldn't do is accept Company A's offer with the intention of backing out of it later if Company B comes through. Company A will have turned their other candidates loose by that point, as well as invested time and money in preparing for your arrival, so reneging on your acceptance would burn that bridge to a crisp. You'd also risk damaging your reputation in your industry, because people talk and you never know when that will come back to haunt you. Assume that once you accept an offer, you'll need to keep your word.
In other words, do what you can to expedite things, but know that you might need to decide if you're willing to end up without any job offers to see the process through with your favored company.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.