Unconventional superconductors are classified according to the symmetry properties of the Cooper pair wavefunction. In centrosymmetric materials, the spin component of the electronic wavefunction is either even (spin-singlet) or odd (spin-triplet) under fermion spin exchange. Within the spin-triplet category, a distinction is made between unitary and nonunitary states, the latter characterized by unequal contributions from up and down spins. Most candidates for spin-triplet superconductivity are found in heavy-fermion and ferromagnetic superconductors. The further classification of the superconducting order parameter is governed by the material’s crystal point group symmetry, leading to diverse spin structures within spin-triplet states. A Zeeman field couples to the electrons’ spin, whose response provides, thus, a direct probe of the spin structure of the order parameter. This thesis focuses on two specific responses of spin-triplet superconductors. First, we analyze the spin susceptibility of generic nonunitary states in the zero-field limit. Second, we investigate the critical field of unitary states, i.e. a finite field effect. The spin susceptibility plays a key role in characterizing the symmetry of the order parameter in unconventional superconductors. Among them, nonunitary spin-triplet superconductors spontaneously break time-reversal symmetry and exhibit a non-degenerate two-band spectrum. These states have garnered significant interest in the context of the search for topological superconductivity. While the spin-susceptibility allows for a clear distinction between spin-singlet and unitary spin-triplet states, surprisingly, so far, the case of nonunitary states has not been explored. We present a general expression for the spin susceptibility in nonunitary triplet superconductors, derived within a single-band model of non-magnetic, centrosymmetric materials with strong spin-orbit coupling. Furthermore, we use our results to assess experimental claims of nonunitary triplet superconductivity. We then explore the phase diagram of spin-triplet superconductors under the influence of a Zeeman field. We find that, for a given order parameter symmetry and generic field orientation, the paramagnetic limit for superconductivity diverges at low temperatures. Additionally, we identify a range of temperatures where the transition between normal and superconducting phases becomes first-order. In cases where two tricritical points exist along the transition line, a first-order phase transition between two superconducting phases may arise near the lower-temperature tricritical point. These findings are discussed in the context of the anisotropy of the upper critical field in UPt₃, a potential spin-triplet superconductor, considering both paramagnetic and orbital effects.