Which ingredient is used to thicken crème pâtissière?

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The thickening in crème pâtissière comes mainly from starch. When you heat milk with sugar, eggs, and starch (often cornstarch), the starch granules absorb liquid and swell in a process called gelatinization. This creates the viscous, smooth, glossy texture that defines a proper pastry cream and helps it hold its shape as it cools. The eggs contribute richness and some additional body through protein coagulation, but without starch the mixture would stay too runny and lack that cohesive custard texture. Gelatin would make the cream set differently and can tone down the smoothness, while pectin is used for jams and gels, not custards. Flour can thicken, but it tends to give a heavier, cloudier texture with a floury taste, whereas starch gives the classic light, silky cream.