This U.S. Bureau of Mines paper presents a brief overview of current bump mechanics theories and pillar design methodologies, and relates these concepts to experiences at two mines in a north-central Utah coalfield where different pillar designs were used to control mountain bumps. Experiences gained at the first mine demonstrated the successful implementation of a two-entry, 9.8-M (32-ft)-wide, yield-pillar design. A Bureau field study quantified the timing of chain pillar yielding and resulting load transfer from the gateroad. In-mine pillar response, although apparently sensitive to site- specific conditions, compared favorably to estimates derived using two yield-pillar design methods. A second study, conducted at another mine located in the same district, but subjected to different geologic conditions, documents in the unsuccessful attempts to employ progressively narrower three-entry pillar designs based on successes achieved at the first mine site. This second mine never achieved a true yield-pillar design. Analysis concluded that yield pillars less than 6.1 M (20 ft) wide and abutment pillars ranging between 30.5 M (100 ft) and 39.6 M (130 ft) square would be required to achieve a stable gateroad design. However, results of a field study conducted on a two-entry, 36.6-M (120-ft)-wide abutment pillar concluded that abutment pressures from the first panel overrode the pillar, and that a still larger pillar may be required to preclude bumps in the tailgate during second panel mining.