❖ Species, strain and breed of animals ❖ Sex, age, size ❖ Maintained singly or in groups ● Ability to form social groups through sight, smell, contact ❖ Design and construct of housing ❖ Availability and suitability of enrichments ❖ Project goals and experimental design ❖ Intensity of animal manipulation and invasiveness ❖ Presence of hazardous materials ❖ Duration of holding period