This paper presents a Lorentz-violating extension of the Standard Model, incorporating both CPT-even and CPT-odd terms. The extension is derived from a fundamental theory with Lorentz-covariant dynamics, where spontaneous Lorentz violation occurs. The resulting theory maintains gauge invariance, energy-momentum conservation, and observer Lorentz invariance, while breaking particle Lorentz invariance. The theory is hermitian, power-counting renormalizable, and preserves microcausality and energy positivity. Spontaneous symmetry breaking to the electromagnetic U(1) is maintained, though the Higgs expectation value is slightly shifted and the Z^0 field acquires a small expectation. A general Lorentz-breaking extension of quantum electrodynamics (QED) is extracted, and experimental tests are considered, including modifications to photon behavior such as vacuum birefringence. Radiative corrections to the photon propagator are examined, showing compatibility with spontaneous Lorentz and CPT violation at Planck-scale levels. The paper also discusses experimental tests of Lorentz and CPT symmetry, including neutral-meson oscillations, Penning trap QED tests, and baryogenesis. The extended QED includes CPT-even and CPT-odd terms, with the pure-photon sector showing potential for experimental tests. The paper concludes with a discussion of the implications of Lorentz violation for gravity and the challenges of detecting such effects in experiments.This paper presents a Lorentz-violating extension of the Standard Model, incorporating both CPT-even and CPT-odd terms. The extension is derived from a fundamental theory with Lorentz-covariant dynamics, where spontaneous Lorentz violation occurs. The resulting theory maintains gauge invariance, energy-momentum conservation, and observer Lorentz invariance, while breaking particle Lorentz invariance. The theory is hermitian, power-counting renormalizable, and preserves microcausality and energy positivity. Spontaneous symmetry breaking to the electromagnetic U(1) is maintained, though the Higgs expectation value is slightly shifted and the Z^0 field acquires a small expectation. A general Lorentz-breaking extension of quantum electrodynamics (QED) is extracted, and experimental tests are considered, including modifications to photon behavior such as vacuum birefringence. Radiative corrections to the photon propagator are examined, showing compatibility with spontaneous Lorentz and CPT violation at Planck-scale levels. The paper also discusses experimental tests of Lorentz and CPT symmetry, including neutral-meson oscillations, Penning trap QED tests, and baryogenesis. The extended QED includes CPT-even and CPT-odd terms, with the pure-photon sector showing potential for experimental tests. The paper concludes with a discussion of the implications of Lorentz violation for gravity and the challenges of detecting such effects in experiments.