Individual differences in behavior lead to wide variability in working dog suitability, and are the primary reason for rejection or early release. Behavioral suitability of a working dog is shaped by interactions with its environment during early development and specialized training. Understanding how aspects of development and training affect a working dog’s performance is critical for practitioners to effectively evaluate and treat behavioral concerns in working dogs. This article provides an overview of critical aspects of puppy development that influence future behavior, and reviews important features of training that influence a dog’s ability to learn and perform its designated task.