The Torah commandment to eat maror on the night of the 15th of Nisan is contingent upon the eating of the Paschal sacrifice, as it states: “They shall eat it with matzot and merorim [plural of maror]” (Bamidbar 9:11). Since we are unable to offer the Paschal sacrifice today, the mitzva to eat maror is now rabbinic (Pesaḥim 120a). We dip the maror in ḥaroset in order to counteract its bitterness, for the maror symbolizes the bitter enslavement. According to many, the idea is not to dip the maror deep into the ḥaroset, but just to barely touch the ḥaroset. If a bit of ḥaroset sticks to the maror, we shake it off the maror, because maror should not be eaten with the sweet ḥaroset (SA 475:1, MB 13 and Kaf Ha-ḥayim 23 ad loc.). After this we recite the berakha “Who sanctified us with His mitzvot and commanded us concerning eating maror” (“asher kideshanu be-mitzvotav ve-tzivanu al akhilat maror”) and eat a kezayit of maror.
The Sages enumerate five types of maror: ḥazeret, tamḥa, ḥarḥavina, ulshin, and maror. Today we are only familiar with two of these: ḥazeret, which is lettuce, and tamḥa, which is horseradish. The Sages stated that the choicest type of maror is lettuce, and its Hebrew name (“ḥasa”) even alludes to the fact that God has mercy (“ḥas”) upon us. They also tell us that the Egyptian enslavement was like maror: just as maror begins soft and ends hard – the stalk starts off soft and later hardens – so too the enslavement began “softly” and became harder and bitterer with time (Pesaḥim 39a).
Some have questioned whether we fulfill the obligation of maror with today’s lettuce, which does not have a bitter taste. Some say that one can fulfill his obligation only with lettuce that has become somewhat bitter. In practice, though, one can fulfill his obligation with lettuce even if it is not bitter, because this was the nature of the bondage: initially the Egyptians enslaved us with gentle words and paid us for our labor, while gradually intensifying the work until it was bitter as gall. The Yerushalmi (Pesaḥim 2:5) likewise tells us that lettuce starts out sweet and then becomes bitter. Thus, it is the custom of all Jews to be scrupulous about this mitzva by using lettuce for maror, because maror need not be bitter when it is eaten; rather, it must be of a species that eventually becomes bitter. Some people add a bit of horseradish to their lettuce so that they taste some bitterness.
Since lettuce often contains bugs, it should be cleaned and checked thoroughly on Erev Pesaḥ. Nowadays there are ways to grow bug-free lettuce, and it is best to use such lettuce in order to avoid potentially violating the prohibition against eating bugs (Peninei Halakha: Kashrut 24:9-10).
One must eat a kezayit (half an egg’s volume) of maror, and one may estimate this size by sight. As we have learned, some customarily calculate a kezayit by weight, which is about 25 grams. However, with lettuce there is very little difference between a weight- and volume-based kezayit.
Another opinion appears in a beraita in Pesaḥim 39a: Any bitter vegetable that exudes latex and has a blanched complexion (i.e., it is whitish-green) is maror. The Rishonim debated this: according to Ri’az, Maharam Ḥalawa, and others, any vegetable that has these characteristics is indeed maror. According to Smak, on the other hand, these characteristics are just features common to the five types of vegetables mentioned in the Mishna, but no vegetable outside what is listed in the Mishna is acceptable for maror. Rif and Rambam do not mention these characteristics, either because they maintain that only the five vegetables listed in the Mishna are acceptable, and these characteristics consequently are meaningless, or because they maintain that we are not experts in identifying these characteristics, so we have no ability to determine what other vegetables are fit for maror. Practically, one who does not have lettuce or horseradish should use another vegetable that has these characteristics, but should not recite a berakha over it, in case we are not adept at identifying these characteristics or it is not one of the five acceptable species. This is the ruling in MB 473:46 and BHL ad loc.