1. This is similar to conspiracy, but need not be. Mainly, it consists of inducing other people to breack their contracts, or making it impossible for them to fulfill their obligations.
Lumley v. Gye 1853 - singer induced to break her contract.

2. Malice is not required anymore, but knowledge of, or at least recklessness in respect to, the existence of the contract is essential.

3. The interference must be direct, except when unlawful means are employed:
Daily Mirror Newspapers Ltd. v. Gardner 1968 - union official illegally induced their members (newspaper retailers) to stop taking supplies from the the plaintiff.

4. Interference need not amount to actual breach: Torquay Hotel Co. Ltd. v. Cousins 1969 - union officials threatened to stop oil supplies; this would not have amounted to breach because of an exclusion clause.

5. Where the interference occurs by inducement, the latter element must consist in some sort of persuasion; advice is not enough.

6. Defenses
a) Lawful justification (of the tort; not the act, see above): the acting being prompted by impersonal and disinterested motives is not enough, neither is it a defence to prove that one has acted in his own, or his association's, interests.
But compare Brimelow v. Casson 1924 - actors' association boycotting a theatrical manager who paid too little.
b) Acts in contemplation of furtherance of a trade dispute; except where the acting is prompted by the fact or belief that an employer has employed, or might do so, a person which is not a member of the union (against the 'closed shop' principle).