We present the JWST Emission-Line Survey (JELS), a JWST imaging programme exploiting the wavelength coverage and sensitivity of the Near-Infrared Camera (NIRCam) to extend narrow-band rest-optical emission-line selection into the Epoch of Reionization (EoR) for the first time, and to enable unique studies of the resolved ionized gas morphology in individual galaxies across cosmic history. The primary JELS observations comprise ∼ 4.7 μm narrow-band imaging o v er ∼ 63 arcmin ² designed to enable selection of H α emitters at z ∼ 6 .1 and a host of no v el emission-line samples, including [O III ] ( z ∼ 8.3) and P aschen α/β ( z ∼ 1.5 / 2. 8). F or the F466N/F470N narrow-band observations, the emission-line sensitivities achieved are up to ∼ 2 ×more sensitive than current slitless spectroscopy surv e ys (5 σ limits of 0.8–1.2 ×10 ⁻¹⁸ erg s ⁻¹ cm ⁻² ), corresponding to unobscured H α star formation rates (SFRs) of 0.9–1.3 M _☉ yr ⁻¹ at z ∼ 6 . 1, extending emission-line selections in the EoR to fainter populations. Simultaneously, JELS also adds F200W broad-band and F212N narrow-band imaging (Hα at z ∼ 2 . 23) that probes SFRs ≳ 5 × fainter than previous ground-based narrow-band studies ( ∼ 0 . 2 M _☉ yr ⁻¹ ),offering an unprecedented resolved view of star formation at cosmic noon. We present the detailed JELS design, key data processing steps specific to the survey observations, and demonstrate the exceptional data quality and imaging sensitivity achieved. We then summarize the key scientific goals of JELS, demonstrate the precision and accuracy of the expected redshift and measured emission-line recovery through detailed simulations, and present examples of spectroscopically confirmed H $\alpha$ and [O iii] emitters discovered by JELS that illustrate the novel parameter space probed.