The primary objective of this investigation was to identify which components of endurance training (e.g., modality, duration, frequency) are detrimental to resistance training outcomes. A meta-analysis of 21 studies was performed with a total of 422 effect sizes (ESs). Criteria for the study included were (a) compare strength training alone to strength plus endurance training (concurrent) or to compare combinations of concurrent training; (b) the outcome measures include at least one measure of strength, power, or hypertrophy; and (c) the data necessary to calculate ESs must be included or available. The mean ES for hypertrophy for strength training was 1.23; for endurance training, it was 0.27; and for concurrent training, it was 0.85, with strength and concurrent training being significantly greater than endurance training only. The mean ES for strength development for strength training was 1.76; for endurance training, it was 0.78; and for concurrent training, it was 1.44. Strength and concurrent training was significantly greater than endurance training. The mean ES for power development for strength training only was 0.91; for endurance training, it was 0.11; and for concurrent training, it was 0.55. Significant differences were found between all the 3 groups. For moderator variables, resistance training concurrently with running, but not cycling, resulted in significant decrements in both hypertrophy and strength. Correlational analysis identified significant negative relationships between frequency (-0.26 to -0.35) and duration (-0.29 to -0.75) of endurance training for hypertrophy, strength, and power. Significant relationships (p < 0.05) between ES for decreased body fat and % maximal heart rate (r = -0.60) were also found. Our results indicate that interference effects of endurance training are a factor of the modality, frequency, and duration of the endurance training selected.