A process for preparing a functionalised polyolefin is described. The process comprises the step of reacting: (i) a polyolefin, (ii) a compound containing a functional group, and (iii) a coagent compound having Formula I: Xn—Y—Zm  (I) wherein: Y is a core moiety; Z is a moiety containing a carbon-carbon double bond or a carbon-carbon triple bond; X is a moiety different than Z and the bond between X and Y may be a single bond or a double bond; n is a whole number greater than or equal to 0; m is a whole number greater than or equal to 2; and m+n equals a number up to the valence of Y; wherein: (a) when m is 2 and n is 0, the process comprises initially reacting the polyolefin and the coagent to form a product that is then reacted with the compound containing the functional group, and (b) the compound containing a functional group is substantially inert in free radical-mediated polymerization reactions and in free radical-mediated grafting reactions. The invention also relates to a functionalized polyolefin produced by such a process. The polyolefins, particularly high molecular weight polyolefins (Mn>10,000), can be extensively modified with functional groups (e.g., thiols, epoxides, anhydrides, carboxylic acids, amines, amides, boron-containing compounds, silicon-containing compounds, cyano-containing compounds, sulfates, sulfonates, sulfites, esters, thioesters, dithioesters, ether, halides, phosphates, phosphonates, phosphines, phosphites and other compounds containing polar moieties) without altering molecular weight thereby rendering the functionalised polyolefins useful in a wide range of applications.