a method and system of forming a lateral wellbore in a time and trip saving manner using a mill / drill to locate and place a casing window . in one aspect of the invention , a lateral wellbore is drilled with liner which is subsequently left in the lateral wellbore to line the sides thereof . in another aspect , the mill / drill is rotated with a rotary steerable system and in another aspect , the mill / drill is rotated with a downhole motor or a drill stem .