The core ruling is that one may never leave Eretz Yisrael for chutz la'aretz except for three purposes — to study Torah, to marry, or to engage in commerce — and even then one must return, as Rambam, Melachim and Wars 5:9 states; the Gemara in Bava Batra 91a adds that one may not leave even during famine unless grain prices have doubled, and even then only if one cannot otherwise obtain food, while Ruth Rabbah 1:4 and Rashi (Bereishit 46:3) and the Sforno (Bereishit 26:1) illustrate the anguish and caution associated with such departures.
Dwelling in Eretz Yisrael carries severe weight: Rambam, Melachim and Wars 5:12 and Ketubot 110b teach that one should always live in Eretz Yisrael even among a non-Jewish majority rather than in chutz la'aretz even among a Jewish majority, for one who leaves is considered like an idolater, a comparison drawn from Rashi (Shmuel I 26:19) and quoted likewise by Kuzari 2:22 and Sefer Charedim, Commandments Dependent on the Land of Israel 1:14.
Within the permitted exceptions, a kohen may leave and even become ritually impure abroad in order to study Torah or marry, as taught in Avodah Zarah 13a and Avodah Zarah 13a:7-8, and codified by Rambam, Mourning 3:14; but Tosafot on Avodah Zarah 13a qualifies that this leniency applies only when one intends to return to Eretz Yisrael, and only for the weightier mitzvot of Torah study and marriage, not for lesser mitzvot generally.
The matrimonial dimension of this halacha is spelled out in Shulchan Arukh, Even HaEzer 75:3-5 and the underlying mishnah in Ketubot 110b: a wife is compelled not to leave Eretz Yisrael and a husband who insists on leaving must divorce her with her ketubah, while one who refuses to ascend to Eretz Yisrael from abroad forfeits the marriage; Rashi on Ketubot 110b:14-15 clarifies the terms "ascending" and "leaving," and Tosafot on Ketubot 110b:14-15 records Rabbeinu Chaim's view that nowadays this compulsion may not fully apply, due to the dangers of travel and the difficulty of properly observing the land-dependent mitzvot.
Related sugyot sharpen the scope of the prohibition: Ketubot 111a extends the principle that leaving Eretz Yisrael for Bavel, or Bavel for other lands, is likewise restricted, and Kiddushin 31b records Rabbi Yochanan ruling to Rav Asi that leaving Eretz Yisrael is forbidden even to greet one's mother, leaving the question of true emergencies unresolved in that passage; Gittin 8b meanwhile discusses the sanctity-status of Syria as a zone bridging Eretz Yisrael and chutz la'aretz, relevant to defining the boundary of the prohibition.
Among the Acharonim, Pitchei Teshuvah, Even HaEzer 75:6 cites a dispute over whether there remains a present-day obligation to ascend to Eretz Yisrael, invoking the Ramban's count of the mitzvah against Rabbeinu Chaim's concern about fulfilling land-dependent mitzvot properly, and Chatam Sofer, Yoreh De'ah 234 frames the prohibition as rooted in the land's inherent sanctity rather than solely in the mitzvot tied to it. Sdei Chemed, Maarechet Alef 246 further limits the Torah-level prohibition specifically to returning to Egypt from Eretz Yisrael, suggesting that departure to other lands may only be rabbinically restricted.
Mishnah Berurah 531:14 applies these distinctions practically, ruling that one who leaves Eretz Yisrael for chutz la'aretz may only do so for a matter considered a mitzvah — such as earning a livelihood or visiting a friend — and not for mere touring, this difference affecting even unrelated halachot like preparing to cut one's hair before a festival.
Among modern poskim, Peninei Halakhah, The Nation and the Land 3:9 summarizes that the Rambam permits leaving only for Torah study, marriage, or commerce, with return required, though extended departure is allowed during severe famine; B'Mareh HaBazak Volume IV 140:7-9 cites Rav Shaul Yisraeli's ruling that departure without intent to return is forbidden, while departure intending to return is permitted for any purpose, even commerce, study trips, or family vacations when comparable options are unavailable in Eretz Yisrael. Gray Matter III, Israel, Must Diaspora Jews Make Aliyah? records Rav Moshe Feinstein's reasoning that since the Rambam only forbids leaving rather than forbidding residence abroad, there is no absolute obligation to move to Israel, though one must weigh Rabbeinu Chaim Cohen's concern about properly fulfilling land-dependent mitzvot. Maamar Mordechai Hilchot Shabbat, Chelek 1 4:6 likewise reviews the Rambam's conditions for leaving and affirms that one who descends from the land is considered as though worshipping idolatry.
Shaarei Halacha uMinhag, Choshen Mishpat 31 holds that leaving Eretz Yisrael is permitted only for Torah study or marriage, stressing that the Torah study must be pursued in earnest, implying that one undertaking such a journey should actually study while there.
Teshuvos V'hanhagos (Teshuvos V'hanhagos 2:565) addresses whether a kohen may travel to Eretz Yisrael while intending to return to chutz la'aretz, and cites the Yerushalmi's position that a kohen who has reached Eretz Yisrael may not leave again, even after only a temporary visit.
Tzitz Eliezer (Tzitz Eliezer 20:43) questions whether David's men left Eretz Yisrael voluntarily, concluding that the sages' statement to David about Israel's need for sustenance does not itself constitute proof either way.
Minchas Yitzchak (Minchas Yitzchak 10:108) surveys the sanctity of Eretz Yisrael as the basis for the prohibition on leaving, weighing whether the restriction flows from the land's own holiness, from the mitzvot tied to it, or from the sanctity of Torah, and distinguishes leaving for other lands, for places of Torah study, and for times of hardship or high prices, also citing views that the principle permitting ascent applies even in the present era.
Mishneh Halachos (Mishneh Halachos 11:431) rules that leaving Eretz Yisrael merely for travel remains forbidden even today, year-round and not limited to Chol HaMoed, while departure for a mitzvah purpose — such as greeting a parent, teacher, or friend — or to earn a livelihood is permitted.