A method of radioiodination of a biological targeting moiety, said method comprising: (i) provision of a compound of Formula: (ii) reaction of said compound of Formula with a compound of Formula (IIa) or Formula (IIb): I*-Y 1 -L 2 -N 3 (IIa) I*-Y 1 -L 3 -C≡N + -O - (IIb) in the presence of a click cycloaddition catalyst, to give a conjugate of Formula (IIIa) or (IIIb) respectively: wherein: I* is a radioisotope of iodine suitable for in vivo imaging, chosen from 123 I, 124 I or 131 I; L 1, L 2 and L 3 are each independently a linker group which may be present or absent, wherein said linker group independently comprises a group of formula - m - wherein each A is independently -CR 2 -, -CR=CR-, -C=C-, -CR 2 CO 2 -, -CO 2 CR 2 -,-NRCO-, -CONR-, -NR(C=O)NR-, -NR(C=S)NR-, -SO 2 NR-,-NRSO 2 -, -CR 2 OCR 2 -, -CR 2 SCR 2 -, -CR 2 NRCR 2 -, a C 4-8 cycloheteroalkylene group, a C 4-8 cycloalkylene group, a C 5-12 arylene group, or a C 3-12 heteroarylene group, an amino acid, a sugar or a monodisperse polyethyleneglycol building block; wherein each R is independently chosen from: H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 1-4 alkoxyalkyl or C 1-4 hydroxyalkyl; and m is an integer of value 1 to 20; Y 1 is -Ar 1 -or-X 1 -; where -Ar 1 - is C 3-10 arylene, and -X 1 - is -CH=CH-(CH 2) n -(Ar 1) j - or -CH=CH-(Ar 1) j -(CH 2) n -where j is 0 or 1, and n is an integer of value 0 to 4, wherein when Y 1 is -X 1 -, I* is attached to -CH=CH terminus of X 1; BTM is the biological targeting moiety, which comprises: a 3-80 mer peptide, peptide analogue, peptoid or peptide mimetic which may be a linear or cyclic peptide or combination thereof; a single amino acid; an enzyme substrate, enzyme antagonist enzyme agonist (including partial agonist) or enzyme inhibitor; receptor-binding compound (including a receptor substrate, antagonist, agonist or substrate); an oligonucleotide, or an oligo-DNA or oligo-RNA fragment, where the compound of Formula is generated by deprotection of a compound of Formula (Ia): wherein M 1 is an alkyne-protecting group. (i) provision of a compound of Formula: (ii) reaction of said compound of Formula with a compound of Formula (IIa) or Formula (IIb): I*-Y 1 -L 2 -N 3 (IIa) I*-Y 1 -L 3 -C≡N + -O - (IIb) in the presence of a click cycloaddition catalyst, to give a conjugate of Formula (IIIa) or (IIIb) respectively: wherein: I* is a radioisotope of iodine suitable for in vivo imaging, chosen from 123 I, 124 I or 131 I; L 1, L 2 and L 3 are each independently a linker group which may be present or absent, wherein said linker group independently comprises a group of formula - m - wherein each A is independently -CR 2 -, -CR=CR-, -C=C-, -CR 2 CO 2 -, -CO 2 CR 2 -,-NRCO-, -CONR-, -NR(C=O)NR-, -NR(C=S)NR-, -SO 2 NR-,-NRSO 2 -, -CR 2 OCR 2 -, -CR 2 SCR 2 -, -CR 2 NRCR 2 -, a C 4-8 cycloheteroalkylene group, a C 4-8 cycloalkylene group, a C 5-12 arylene group, or a C 3-12 heteroarylene group, an amino acid, a sugar or a monodisperse polyethyleneglycol building block; wherein each R is independently chosen from: H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 1-4 alkoxyalkyl or C 1-4 hydroxyalkyl; and m is an integer of value 1 to 20; Y 1 is -Ar 1 -or-X 1 -; where -Ar 1 - is C 3-10 arylene, and -X 1 - is -CH=CH-(CH 2) n -(Ar 1) j - or -CH=CH-(Ar 1) j -(CH 2) n -where j is 0 or 1, and n is an integer of value 0 to 4, wherein when Y 1 is -X 1 -, I* is attached to -CH=CH terminus of X 1; BTM is the biological targeting moiety, which comprises: a 3-80 mer peptide, peptide analogue, peptoid or peptide mimetic which may be a linear or cyclic peptide or combination thereof; a single amino acid; an enzyme substrate, enzyme antagonist enzyme agonist (including partial agonist) or enzyme inhibitor; receptor-binding compound (including a receptor substrate, antagonist, agonist or substrate); an oligonucleotide, or an oligo-DNA or oligo-RNA fragment, I* is a radioisotope of iodine suitable for in vivo imaging, chosen from 123 I, 124 I or 131 I; L 1, L 2 and L 3 are each independently a linker group which may be present or absent, wherein said linker group independently comprises a group of formula - m - wherein each A is independently -CR 2 -, -CR=CR-, -C=C-, -CR 2 CO 2 -, -CO 2 CR 2 -,-NRCO-, -CONR-, -NR(C=O)NR-, -NR(C=S)NR-, -SO 2 NR-,-NRSO 2 -, -CR 2 OCR 2 -, -CR 2 SCR 2 -, -CR 2 NRCR 2 -, a C 4-8 cycloheteroalkylene group, a C 4-8 cycloalkylene group, a C 5-12 arylene group, or a C 3-12 heteroarylene group, an amino acid, a sugar or a monodisperse polyethyleneglycol building block; Y 1 is -Ar 1 -or-X 1 -; where -Ar 1 - is C 3-10 arylene, and -X 1 - is -CH=CH-(CH 2) n -(Ar 1) j - or -CH=CH-(Ar 1) j -(CH 2) n -where j is 0 or 1, and n is an integer of value 0 to 4, wherein when Y 1 is -X 1 -, I* is attached to -CH=CH terminus of X 1; where -Ar 1 - is C 3-10 arylene, and -X 1 - is -CH=CH-(CH 2) n -(Ar 1) j - or -CH=CH-(Ar 1) j -(CH 2) n -where j is 0 or 1, and n is an integer of value 0 to 4, wherein when Y 1 is -X 1 -, I* is attached to -CH=CH terminus of X 1; BTM is the biological targeting moiety, which comprises: a 3-80 mer peptide, peptide analogue, peptoid or peptide mimetic which may be a linear or cyclic peptide or combination thereof; a single amino acid; an enzyme substrate, enzyme antagonist enzyme agonist (including partial agonist) or enzyme inhibitor; receptor-binding compound (including a receptor substrate, antagonist, agonist or substrate); an oligonucleotide, or an oligo-DNA or oligo-RNA fragment, where the compound of Formula is generated by deprotection of a compound of Formula (Ia): wherein M 1 is an alkyne-protecting group.