The state-level rules every owner and walker in Illinois should know. Local leash lengths, licensing and off-leash rules are set by each city — find those on the city pages below.
Illinois is a strict-liability state, and its Animal Control Act defines "owner" to include anyone who has the dog in their care or acts as its custodian — so strict liability can attach to a walker or sitter.
Illinois' Animal Control Act (510 ILCS 5/16) is broad strict liability: if a dog without provocation attacks, attempts to attack, or injures a person who is peaceably in a place they may lawfully be, the owner is liable for the full amount of the injury — no prior bite and no knowledge required. It covers injuries, not just bites (a dog knocking someone over counts). The only real defenses are provocation and trespass.
The walker-critical part is the statutory definition of owner (510 ILCS 5/2.16): a person with a right of property in the animal, or who keeps or harbors it, or who has it in their care, or acts as its custodian, or who knowingly permits it to remain on premises they occupy. Legal commentary is explicit that dog-sitters and temporary caretakers can face liability — when you are walking or sitting a client's dog, you have it in your care and act as its custodian, so you are an owner under the Act.
On breed, the Act says vicious dogs shall not be classified in a manner specific to breed — though home-rule municipalities can pass their own breed rules. Leash and confinement rules are local (most cities require leashing in public), and rabies vaccination is required statewide (510 ILCS 5/8). The dangerous and vicious-dog process (5/15) requires enclosure, muzzle, signage, and insurance for a vicious designation.
Illinois uses modified comparative negligence with a 51% bar (735 ILCS 5/2-1116). The personal-injury limit is two years.
This is general information about Illinois law, not legal advice. Confirm current rules with the official state and municipal sources.