One who missed a word or part of the reading?
The reader must read every word of the Megillah. If he skipped a verse, or even one word, or even one letter, he and the congregation do not fulfill their obligation. The same applies for the listener; if he did not hear even one word or even one letter, he has not fulfilled his obligation. [One must thus repeat the reading. Nevertheless, one is not to repeat the blessing prior to the reading.]
Reading the missed word from a Chumash or from memory: The missed word may be read even from a Chumash or memory. This applies even if one missed the first and last verse of the Megillah, or an entire episode. If, however, majority of the Megillah was read from a Chumash or memory, the reading is invalid.
Reading the skipped word in the correct order: One who reads the Megillah in the wrong order does not fulfill his obligation. Thus, if one skipped a [letter, word or] verse and continued reading, he must go back to the skipped verse [word or letter] and read the rest of the Megillah from there until the end. [It is to be read loud enough for one to hear his voice but not so loud that others can hear him.]
One who walked into Shul the middle of the reading: Due to the above, one who walks into Shul in middle of the reading of the Megillah, cannot fulfill his obligation by simply listening to the rest of the reading and then reading the parts he missed to himself. Rather he must repeat the reading from the beginning.
When Haman’s name is recited: Due to the noise level made during the recitation of Haman each person is to read to himself from a Megillah or Chumash one or two verses in proximation to “Haman”, just in case the Chazan has already continued reading and one is unable to hear. [In many communities today the custom is for the Chazan to remain silent until all the noise in the crowd has stopped.]
Reading along in a Chumash or invalid Megillah:
One who does not have a Kosher Megillah available is to read along in a Chumash or printed Megillah.
One who missed a word of the Chazan: If one did not hear a word from the Chazan he must verbally read that missed word. He must continue reading from the missed word until he catches up to the Chazan’s reading, in which case he is to stop verbalizing the words and listen to the reading.
If one was thinking of other matters during the Megillah reading does he fulfill his obligation?
If one heard every word of the Megillah he fulfills his obligation even if he did not have intention by every word. However some Poskim rule that this only refers to one who did not pay attention to the meaning of the words, however if one was thinking of other matters while hearing the words then it is considered as if he did not hear those words. For this reason it is of importance for one to verbally read along in a Kosher Megillah, as in such a case, even if he does not pay attention to the words, he still fulfills his obligation. Others suggest that even if one does not verbalize the words silently together with the Chazan, if he is following along inside in his mind, then even if his mind wanders, he fulfills his obligation.
 M”A 690/4 and 7 and 15; Beis Yosef 692; Ran and Rashba 467; Peri Chadash; P”M 690 A”A 15; Chayeh Adam 154/16; M”B 690/5 [majority of Poskim] and 690/19 [according to some Poskim] and 690/48 and 692/9 [plainly rules is not Yotzei]; See Biur Halacha 690 “Ein Midakdikin”; Levush brought in Kaf Hachaim 689/7; 692/22; Kaf Hachaim 690/12 and 15-16 and 36 and 82
Other Opinions: Some Poskim rule the reading is valid even if the reader or listener missed many words. [There are opinions in the Talmud who hold one is only required to hear the Megillah starting from Ish Yehudi. [Shaar Hatziyon 690/5] The Riaz [brought in Shiltei Giborim; Kneses Hagedola; M”A 690/4; P”M 690 M”Z 4] rules that the reading is valid even if the reader or listener missed many words, so long as the following words or letters were read: Viyiomei Hapurim Haeilu Lo Yavru; Umatanos Laevyonim.] The M”A ibid negates this opinion. [Kaf Hachaim 690/16; See Biur Halacha 690 “Ein Midakdikin”]
 Michaber 690/3; Kaf Hachaim 690/36
 Mateh Yehuda brought in Kaf Hachaim 690/12
 Rashba 467 brought in M”A ibid; M”B ibid in name of most Poskim
 Chayeh Adam ibid; Kaf Hachaim 690/16
 Must the entire reading be repeated or only from the missed area and onwards? If one finished the reading and removed his mind from the reading, and then discovered that he missed a word, seemingly he must repeat the reading from the beginning. If however one did not remove his mind from the reading then he may continue from the area that he missed. [see Poskim in next footnote]
 Kaf Hachaim 690/16; Chayeh Adam 154/16 brought in Biur Halacha 690 “Ein Medakdikin” [and so seems to be the conclusion of the Biur Halacha, unlike the understanding of Piskeiy Teshuvos 690/5 that the Biur Halacha holds one must repeat with a blessing in all cases]; Lehoros Nasam 9/20
The Biur Halacha ibid writes that according to all the Poskim that rule a missed word invalidates the reading, one must repeat the reading with a blessing. The Chayeh Adam ibid [brought in Biur Halacha ibid] however suspects for the stringent opinion of the Riaz and hence rules that only by words or verses that we know for certain that they must be read according to all, then if they were skipped, one is to read the Megillah again with a blessing. However, the Kaf Hachaim ibid concludes that practically a blessing is never said by all cases as we are unsure which parts are invalid according to all, and Safek Brachos Lihakel. Vetzaruch Iyun Gadol as to why at all a blessing would ever be required even according to the stringent opinions, as the law is that the reading is valid if it was read inconsecutively [Biseirugin] and hence the listener would only be required to hear the reading from the area he missed and onwards, in continuation of the original blessing. [see 690/5 or Halacha 13 above] This is similar to the blowing of the Shofar inconsecutively in which case the original blessing covers all the blows, even if an interval took place [590/13] Vetzaruch Iyun Gadol! The Lehoros Nasan 9/20 addresses this question and suggests that even according to the stringent opinion a blessing is only required if one removed his mind completely from the previous reading. However, in a case that one did not remove his mind from the reading then according to all one would not be required to repeat the blessing, even if he missed a word that is required to be heard according to all.
 See Halacha 18B and Halacha 24A
 690/6; See Halacha 29!
 Piskeiy Teshuvos 689/7; See Halacha 3 in Q&A; Halacha 18B
 Megillah 18b; Shulchan Gavoa 690/14; Kaf Hachaim 690/42
 M”A 690/19; M”B 690/60; M”B 690/19 in name of Peri Megadim 690 A”A 7; Kaf Hachaim 690/36
 M”B 690/19 in name of Peri Megadim 690 A”A 7; Kaf Hachaim 690/36
The reason: The reason for this is because it is difficult for listeners to hear all the words due to the noise made during Haman. Hence they should read in the missing words from the Chumash and at the very least fulfill their obligation Bedieved. [ibid]
 M”B 690/19; 690/60; Peri Megadim 690 A”A 7
 M”A 690/15; Kaf Hachaim 690/82
 Levushei Serud on M”A ibid; see also Nemukei Orach Chaim 690; Piskeiy Teshuvos 690/4; Eretz Tzevi 1/45; Shearim Hametzuyanim Behalacha 141/13
 This commonly occurs to Torah scholars and business men that their minds wander to Torah thoughts and business dealings when hearing something read for a long period of time. [Nemukei Orach Chaim ibid]
 Nemukei Orach Chaim ibid
 Eretz Tzvi ibid [look along in a Kosher Megillah]; Shearim Hametzuyanim ibid [even in a Chumash this helps, and the above ruling of the Levushei Serud refers to one who was listening without a Chumash or Megillah.]
 P”M 692 M”Z 3; M”B 692/16; Kaf Hachaim 692/39
 The reason: As although Megillah reading is merely a Rabbinical command, perhaps since it is from the words of Kabala it is thus not subject to the rule of Safek Derabanan Lekula. [ibid] Piskeiy Teshuvos 689 footnote 17 suggests that by the night reading one may be lenient being it is not from Kabalah.