Jackie - read your lease - it should state how and when the landlord can gain access.
If he or she perceives there may be a problem, I assume they can enter (say a fire or leak). I would further assume that they should give you a courtesy call if they need to get in for some maintenance issues.
If you feel the landlord is entering without reason or the right to do so, then I'd have a discussion about this right away. You should have the right to "peaceful" existance - and your privacy should be respected.
Once again - read your lease for any verbiage regarding gaining access to your apartment.