Purpose: The purpose of this study was to analyze the implant stability quotient (ISQ) values recorded by three commercially available resonance frequency analysis (RFA) instruments from a large cohort of implants in order to determine their accuracy and agreement with one another both for static measurements of ISQ at a given time and for change in ISQ over time. Materials and Methods: A cohort of n = 210 implants had their primary stability, secondary stability, or both evaluated in both the mesiodistal (MD) and buccolingual (BL) directions by means of ISQ using three different RFA instruments: Osstell ISQ (O_ISQ), Osstell IDx (O_IDX), and the Penguin (PG). ISQ values were recorded both at the time of implant placement and at 3 months postinsertion prior to definitive restoration. All values were tabulated for a blinded statistical analysis using Bland-Altman plots to determine if the outcome values were in agreement both for primary and secondary stability. In addition, a subgroup was evaluated to determine if change in ISQ was also in agreement. An intraclass correlation (ICC) was used to measure the reliability of the measurements for each instrument. Results: Bland-Altman plots confirmed that there was a high agreement for MD values between O_ISQ and O_IDX, with 72.7% of readings being within one ISQ unit and 94.7% within four units. Comparing PG to OISQ, the respective values at one and four units were 15.3% and 82.3%, and comparing PG to O_IDX, the respective values were 16.3% and 85.2%. In general, there was a greater uncertainty in the BL values having wider variability and demonstrating less agreement between instruments, with the percentage of readings falling within four units reducing to 85.9% (O_ISQ vs O_IDX), 72.3% (PG vs O_ISQ), and 74.3% (PG vs O_IDX). For change in ISQ over time, 92.3% of values were in agreement to within four units between instruments O_ISQ and O_IDX in the MD direction and 73% in the BL direction. The respective percentage changes of values in agreement within four units for PG vs O_ISQ were 76.9% and 60.3% and for PG vs O_IDX were 80% and 53.8%. The paired t test from mixed effects revealed that there was a significant difference for mean MD values between PG vs O_ISQ; P = .015 with a mean 0.823 units higher was recorded for PG. Similarly for PG vs O_IDX, P = .008 with a mean 0.871 units higher was recorded for PG. For mean BL values between PG vs O_IDX, P = .000 with a mean 1.161 units higher was recorded for PG, and finally, for O_ISQ vs O_IDX, P = .005 with a mean 0.597 units higher was recorded for O_ISQ. However, the maximum upper and lower bound estimated bias between any two instruments was only 1.86 units and 0.46 units both for PG vs O_IDX in the BL direction, and it is doubtful that this is of clinical relevance even if statistically significant. ICC revealed that for static MD measurements, there was an 85% reliability between all three instruments (range: 79% to 97%). For BL measurements, the reliability value was 66% (range: 69% to 71%). When considering ICC for changes in ISQ values over time in the MD direction, there was a 70% reliability between all three instruments (range: 58% to 94%). For BL measurements, the reliability value was 58% (range: 46% to 91%). Conclusion: Differences exist between instruments to some extent, most notably between the Penguin and the two instruments from Osstell, which showed good agreement to each other. While differences in evaluating ISQ with the PG were statistically significant, they were less than 1.86 units at the upper bound limit, and it is doubtful that this is of clinical relevance. Increased variability and reduced reliability for BL values render these less clinically sound when trying to assess primary stability. Schlagwörter: dental implants, implant stability quotient, ISQ, primary stability, resonance frequency analysis