Objective: Knee buckling, in which a knee gives way during weight-bearing, is common in people with knee pain and knee osteoarthritis (OA), but little is known about the prevalence of sensations of knee instability, slipping or shifting in which the knee does not actually buckle, or of the psychosocial and physical consequences of these symptoms. Design: We asked participants in the Multicenter Osteoarthritis Study (MOST) separately about episodes of knee buckling and sensations of knee instability without buckling in the past 3 months, and assessed fear of falling, poor balance confidence (Activities-specific Balance Confidence (ABC) Scale≤67/100), activity limitation due to concern about buckling, and poor physical function (Western Ontario and McMaster Universities Arthritis Index (WOMAC) physical function≥28/68). We used Poisson regression to estimate prevalence ratios (PRs) for cross-sectional associations of buckling and sensations of instability without buckling with these outcomes, adjusting for confounders. Results: Of 2120 participants (60% female, 40%≥65 years, mean Body mass index (BMI): 31kg/m258), 18% reported buckling, 27% had sensations of knee instability without buckling, and 9% reported both symptoms. Buckling and sensations of instability without buckling were each significantly associated with fear of falling, poor balance confidence, activity limitations, and poor WOMAC physical function. Subjects who reported both buckling and instability without buckling and those with at least two buckling episodes (15%) had the strongest association with poor outcomes. Conclusions: Knee buckling and especially sensations of knee instability without buckling were common and each was significantly associated with fear of falling, poor balance confidence, activity limitations, and poor physical function. © 2014 Osteoarthritis Research Society International.