I'm waiting for an offer letter from Company B despite having already?
Its best to be as honest as possible in these situations, assuming that Company A is honest and open as well. Thus, you should first let Company A know that you are interviewing elsewhere and are waiting to hear other offers. If they work with you on this, then this is the best possible outcome - there may even be negotiating room for you to be open about why Company B is 'in the lead' be it salary, title, etc and a chance for Company A to show you that they can be competitive and to change your mind. If Company A seems dishonest etc, and sets an unreasonable deadline/exploding offer that you have to tell them by, then THEY are the ones initiating aggressive tactics, so its OKAY to then to play the game and accept their offer even if you are still waiting on Company B. Absolutely don't join a company then leave after a month though. Thats about the worst thing you can do. The company wouldn't like it, and you'd probably feel uncomfortable working at a company you were going to quit in a month. Ultimately it comes down to what type of person you are, and this is pretty low. Instead, try to set your official 'start date' back a few weeks so you have time to hear back from Company B before your first day of Company A. If you have to, say you had a vacation planned or other excuse to make sure your start date is pushed back. Its a 'less egregious' offense to 'never start' than to start and quit for a month.