It has become a universally-accepted custom to reciteBarechubefore proceeding to the blessings ofShema. Barechu,likeKaddishandKedushah,cannot be recited unless a quorum—aminyanof ten adult males—is present.Barechuis also said by one who is called upon to read from the Torah. The practice of sayingBarechuis mentioned numerous times in the Talmud. It is even cited in the Mishna (Maseches Berachos 49b) in the names of Rabbi Akiva and Rabbi Yishmael.

The Chazzan says:

Bless Adonoy Who is blessed.

The Congregation responds and the Chazzan repeats:

Blessed is Adonoy, Who is blessed forever and ever.

The two blessings יוֹצֵר אוֹר “He Who forms light” and אַהֲבָה רַבָּה “Great love,” are referred to as the blessings preceding Shema (Maseches Berachos11a). According to Rambam they were formulated by Ezra and his Court (Laws of Shema1:7). You may not interrupt your prayers from this point on until after theShemoneh Esrei*. But you may answer “Amein” to the blessings of הָאֵל הַקָּדוֹשׁ “The holy Powerful One” and שׁוֹמֵעַ תְּפִילָּה “He Who heeds prayer;” say the thrice-repeated קָדוֹשׁ “Holy…” and בָּרוּךְ כְּבוֹד “Blessed is the glory…” ofKedushah; and answer*Barechu.You may also answer אָמֵן יְהֵא שְׁמֵהּ רַבָּא “Amein, May His great Name,” when you hear Kaddish.” You may also answer Amein after the blessings of יוֹצֵר הַמְּאוֹרוֹת, “Who forms the luminaries,” and הַבּוֹחֵר בְּעַמּוֹ יִשְׂרָאֵל בְּאַהֲבָה, “Who chooses His people Israel with love.” For a more detailed exposition of these laws, seeMetsudah Kitzur Shulchan Aruch,Vol. I, pp. 172-176.