This study was performed to compare variations in roots, root canals and pulp space configuration of the maxillary first molar of the Egyptian and Kuwaiti populations by using 2 different methods: laboratory study (in vitro) and clinical radiographic examination (in vivo). Each method was performed separately as follows:-I.Laboratory study (in vitro) :A total of two hundred and seventy extracted permanent, human maxillary first molars teeth were used in this study. They were collected from two different populations and grouped according to the nationality into two main groups:First group: 135 teeth were collected from the outpatient clinic of Endodontic Department, Faculty of Oral and Dental Medicine, Cairo University.Second group: 135 teeth were collected from a several dental clinics and centers in Kuwait.After completion of the mechanical preparation of all teeth, each main two group was divided into three equal subgroups (45 teeth each).The first subgroup was evaluated under the dental operating microscope (DOM).The second subgroup was subjected to clearing and then examined under the binocular stereomicroscope.The third subgroup was subjected to a periapical radiographic examination.