Statute envisages oral or written, express or implied contracts of service, see s. 2 of the Employment Act
Essentially it is a contract, governed by the general rules of contract under common law, hence subject to ordinary rules of construction for contracts.
Law of contract however may be inappropriate for employment cases. Contracts are normally a single transaction documents – limited to a specific transaction. Employment cases concern a continued relationship for a long period of time with elements of trust and confidence.
Employment contracts thus are formed in the same manner as ordinary contracts.
Like other contracts, there must be an offer and an acceptance. The offer and acceptance may be subject to conditions
Wishart v National Association of Citizens’ Advice Bureaux Ltd,  IRLR 393, claimant offered a job ‘subject to receipt of satisfatory references.’ References received were not satisfactory hence the offer was withdrawn. It was held that this was a conditional offer of employment and the defendants had an obligation to consider the references in good faith. The question of being ‘satisfactory’ was subjective.
There must be an intention to create legal relations, consideration and absence of vitiating elements e.g. mistake, illegality, misrepresentation.
If the terms of the contract are designed to avoid payment of income tax, the contract is illegal. See Mohammed Ghias Quereshi & Another Versus Paramount Bank Limited High Court Civil Suit No. 1557 of 1997 (Nairobi)