He is one of the most highly respected Orthodox rabbis in the world and one of the most important leaders of the Lithuanian Haredi world, together with Rabbi Yosef Sholom Eliashiv and Rabbi Shmuel Halevi Wosner.
His beis din (rabbinical court) handles all kinds of matters, such as financial disputes, marriage conflicts, and conversions.
Rabbi Karelitz is a nephew of Rabbi Avrohom Yeshaya Karelitz (the Chazon Ish). The Chazon Ish was a previous rabbinical spiritual leader of Bnei Brak. From the time of the Chazon Ish until Rabbi Nissim Karelitz it was Rav Elazar Shach who was regarded as the pre-eminent leader.
He is also a member of the Vaad Halacha (Jewish legal council) of the Maayanei Hayeshua Hospital of Bnei Brak and the av beis din (head of the rabbinical court) of the Bnei Brak neighborhood of Ramat Aharon.