A compound of Formula or a pharmaceutically acceptable salt thereof, wherein: the antibody moiety is selected from: antibody-L-(CH 2) q - and antibody-L-(aliphatic moiety)-; the aliphatic moiety is selected from a polymer, R P, and a group selected from: -polymer-L-(CH 2) m - and -polymer-L-(CH 2 -CH 2 -O) p -(CH 2) m -; R P is selected from optionally substituted C 1-6 alkyl, optionally substituted C 1-3 alkyl-O-(CH 2 -CH 2 -O) p -(CH 2) m -, and optionally substituted C 3-7 cycloalkyl; each L is independently a linking group; m, p, and q are each independently an integer from 1 to 10; D is a residue of a cytotoxic or chemotherapeutic compound; Z 1 is selected from O, S, and N(R N); Z 3 is selected from O and N(R N), or Z 3 is absent; A is O or N, wherein when A is O then R 3 is absent; R N is selected from H and optionally substituted C 1-6 alkyl; R 3 is selected from H and C 1-6 alkyl, or R 3 and R 1, together with A and the carbon atom to which R 1 is attached, form an optionally substituted 4 to 7 membered aliphatic heterocyclic ring; or R 3 and R 2, together with A, the carbon atom to which R 1 is attached, and the carbon atom to which R 2 is attached, form an optionally substituted 4 to 8 membered aliphatic heterocyclic ring; M A is a self-immolative group having any one of formulae (a)-(i): wherein x denotes a point of attachment to Z 1 and y denotes a point of attachment to z3; R 1 and R 2 are independently selected from the group consisting of hydrogen, optionally substituted C 1-6 alkyl, optionally substituted C 6-10 aryl and optionally substituted 5- to 14-membered heteroaryl; or R 1 and R 2 are joined together with the carbon atoms to which they are attached to form an optionally substituted C 3-7 cycloalkyl ring, an optionally substituted 4 to 7 membered aliphatic heterocyclic ring, an optionally substituted C 6-10 aryl or an optionally substituted 5- to 14-membered heteroaryl; or R 1 and R 2 are joined together to form a ribose ring system; R 7 and R 8 are independently selected from H and C 1-6 alkyl; and E is a cleavable moiety. antibody-L-(CH 2) q - and antibody-L-(aliphatic moiety)-; the aliphatic moiety is selected from a polymer, R P, and a group selected from: -polymer-L-(CH 2) m - and -polymer-L-(CH 2 -CH 2 -O) p -(CH 2) m -; R P is selected from optionally substituted C 1-6 alkyl, optionally substituted C 1-3 alkyl-O-(CH 2 -CH 2 -O) p -(CH 2) m -, and optionally substituted C 3-7 cycloalkyl; each L is independently a linking group; m, p, and q are each independently an integer from 1 to 10; D is a residue of a cytotoxic or chemotherapeutic compound; Z 1 is selected from O, S, and N(R N); Z 3 is selected from O and N(R N), or Z 3 is absent; A is O or N, wherein when A is O then R 3 is absent; R N is selected from H and optionally substituted C 1-6 alkyl; R 3 is selected from H and C 1-6 alkyl, or R 3 and R 1, together with A and the carbon atom to which R 1 is attached, form an optionally substituted 4 to 7 membered aliphatic heterocyclic ring; or R 3 and R 2, together with A, the carbon atom to which R 1 is attached, and the carbon atom to which R 2 is attached, form an optionally substituted 4 to 8 membered aliphatic heterocyclic ring; M A is a self-immolative group having any one of formulae (a)-(i): wherein x denotes a point of attachment to Z 1 and y denotes a point of attachment to z3; R 1 and R 2 are independently selected from the group consisting of hydrogen, optionally substituted C 1-6 alkyl, optionally substituted C 6-10 aryl and optionally substituted 5- to 14-membered heteroaryl; or R 1 and R 2 are joined together with the carbon atoms to which they are attached to form an optionally substituted C 3-7 cycloalkyl ring, an optionally substituted 4 to 7 membered aliphatic heterocyclic ring, an optionally substituted C 6-10 aryl or an optionally substituted 5- to 14-membered heteroaryl; or R 1 and R 2 are joined together to form a ribose ring system; R 7 and R 8 are independently selected from H and C 1-6 alkyl; and E is a cleavable moiety.