Massively multiplexed spectroscopy is revolutionizing our understanding of the universe by enabling efficient, statistically robust surveys across large cosmic volumes. By capturing spectra for thousands of sources simultaneously, it enables us to map large-scale structures, trace galaxy growth in statistically significant samples, and quantify the physical processes driving galaxy evolution as a function of environment. Wide survey areas are also essential to mitigate cosmic variance and place rare or extreme populations within the broader context of the evolving web of matter and dark energy.

Wide-Field Slitless Spectroscopy (WFSS) is critical to this effort by mitigating the selection biases inherent in targeted spectroscopic surveys. Deep, unbiased emission-line samples from HST/ACS and WFC3 grism programs have demonstrated the power of this approach in both blank and cluster fields. JWST’s NIRISS, NIRCam, and MIRI now extend WFSS to fainter fluxes and higher redshifts, enabling integrated and spatially-resolved rest-frame optical spectroscopy into the epoch of reionization. Euclid and Roman WFSS further scale these capabilities to thousands of square degrees, delivering homogeneously selected samples for statistical studies of galaxy evolution, structure formation, and cosmology — without target preselection.

Given the rapid expansion of WFSS modes and the growing volume of legacy data products, there is a pressing need to increase community awareness of both the science cases and the available analysis tools. This workshop will address key science questions related to galaxy evolution, large-scale structure assembly, and time-domain spectroscopy, as well as practical challenges in WFSS calibration and analysis. Through tutorials, demos, talks, posters, and discussions, the workshop aims to address the following:

• Current, upcoming, and future WFSS capabilities are poised to revolutionize our exploration of the universe. Where will the major paradigm shifts occur? What can we achieve now, what will soon become possible, and how should we approach the next generation of capabilities? Crucially, how does WFSS integrate into the broader landscape of astronomical tools? What collaborations with space- and ground-based observations are essential
• With current and upcoming facilities, WFSS will observe large statistical samples of galaxies with high spatial resolution; however, taking advantage of this capability is challenging. What are the high priority science cases for spatially-resolved WFSS and how do we address the challenge in reconstructing spatial information from slitless observations?
• In the era of Euclid and Roman, how can statistical galaxy studies and large-scale structure science advance? What are the key priorities, expected challenges, and connections to cosmology? How do these surveys complement detailed, targeted observations with HST and JWST?
• Time-domain astronomy is entering a new era with Rubin. What discoveries will large-scale time-domain surveys enable, from transients to AGN variability? What science is within reach now, what lies on the horizon, and how can we best connect Rubin’s capabilities with space-based WFSS observations to maximize scientific return?

This Workshop aims to explore these topics in the context of what we have and what we need in terms of WFSS capabilities, tools, and resources for the community. We aim to bring together experts from HST, JWST, Euclid, Roman, and beyond (e.g., CASTOR), and we strongly encourage participation from researchers at all levels of experience. The program will include science and technical talks, hands-on tutorials, and discussion sessions. We invite submissions for tutorials/demos, talks (including review talks), and posters. A white paper summarizing strategies, lessons learned, and future directions will be produced following the meeting.