Muslim scholars teach that Muslims should generally be truthful to each other, unless the purpose of lying is to "smooth over differences" or "gain the upper-hand over an enemy."
There are several forms of lying to non-believers that are permitted under certain circumstances, the best known being taqiyya (the Shia name). These circumstances are typically those that advance the cause of Islam - in some cases by gaining the trust of non-believers in order to draw out their vulnerability and defeat them.