The socialisation period in puppies (3-12 weeks) represents a critical developmental window during which positive exposure to humans, animals, environments, and stimuli establishes lifelong behavioural resilience. Inadequate socialisation is the primary cause of fear-related aggression, noise phobia, and separation anxiety in companion dogs. Puppies raised in barren environments show persistent fearfulness that socialisation after 12 weeks cannot fully reverse. Research demonstrates that puppies attending socialisation classes before 16 weeks show lower adult fear scores. The COVID-19 pandemic created a cohort with documented higher behavioural problems from restricted socialisation opportunities. Best practice emphasises positive reinforcement exposure to diverse stimuli before 12 weeks.