A thermal flow sensor comprising: a first substrate having a first side and a second opposite side; a second substrate having a first side and a second opposite side, said first substrate being connected to said second substrate such that said second side of said first substrate abuts said first side of said second substrate; a third substrate having a first side and a second opposite side, said third substrate being connected to said second substrate such that said second side of said second substrate abuts said first side of said third substrate; and wherein said first, second and third substrate (12, 14, 16) form a multi-layer body structure having side edges extending between a first side of said first substrate and said second side of said third substrate; said second substrate having a groove formed therein so as to form a conduit bounded by said second substrate and said second side of said first substrate and said first side of said third substrate, said conduit having an inlet opening and an outlet opening, each of said openings being formed in said side edges, wherein the conduit is configured to permit fluid to flow through the conduit by entering into the inlet opening and exiting from the outlet opening, and wherein said inlet opening and said outlet opening are disposed solely in said second substrate, the thermal flow sensor further comprising: a heater disposed on said first side of said first substrate opposed to said conduit; and a first temperature sensor disposed on said first side of said first substrate opposed to said conduit. a first substrate having a first side and a second opposite side; a second substrate having a first side and a second opposite side, said first substrate being connected to said second substrate such that said second side of said first substrate abuts said first side of said second substrate; a third substrate having a first side and a second opposite side, said third substrate being connected to said second substrate such that said second side of said second substrate abuts said first side of said third substrate; and wherein said first, second and third substrate (12, 14, 16) form a multi-layer body structure having side edges extending between a first side of said first substrate and said second side of said third substrate; said second substrate having a groove formed therein so as to form a conduit bounded by said second substrate and said second side of said first substrate and said first side of said third substrate, said conduit having an inlet opening and an outlet opening, each of said openings being formed in said side edges, wherein the conduit is configured to permit fluid to flow through the conduit by entering into the inlet opening and exiting from the outlet opening, and wherein said inlet opening and said outlet opening are disposed solely in said second substrate, the thermal flow sensor further comprising: a heater disposed on said first side of said first substrate opposed to said conduit; and a first temperature sensor disposed on said first side of said first substrate opposed to said conduit. a heater disposed on said first side of said first substrate opposed to said conduit; and a first temperature sensor disposed on said first side of said first substrate opposed to said conduit.