1. 4 Tests
   1. P/or must have made P to P/ee to not enforcea legal right
   2. P/ee must have relied on that P
      1. To his detriment: Ajayi v Briscoe
      2. and altered his position as a result: Alan Co ltd v El Nasr & Import (1972)
   3. would be unconscionable for P/or to reniege on his promise
   4. defence not a sword
      1. Combe v Combe (1951)
      2. cf:Re Wyven Developements (1974); Guildford City AFC - Pr Est CAN be a cause of action
2. When does it arise?
   1. Contract between parties
   2. special circumstances
      1. parties negotiating C: per DENNING LJ IN Brinkom Ltd V Car
      2. 'C IRRELEVANT' per Donaldson J in Durham Fancy Goods V. Michael Jackson (1969)
3. Implications
   1. Periodic Payment
   2. Lump Sum Payments