In July of 2012, the Home Office brought in new family visa rules to try to reduce immigration numbers. Under the old rules, there was no official minimum income requirement, but you needed to show approximately £112 left per week after rent and council tax. This money could come from anywhere and you only needed to provide 3-6 months of payslips and bank statements.

Under the new rules, the minimum income requirement for one applicant is £18,600. For the first visa, only the UK citizen's income can count unless the spouse/partner has permission to work in the UK. If the minimum income is not met, savings can be used (£16,000 + 2.5 x shortfall). No financial help from family is allowed. Six months payslips and bank statements, plus letter from employer, job contract and P60 are required for financial evidence.

In addition, evidence of a genuine relationship and suitable accomodation are required.

See also:
Income requirements calculator for UK family visas
Financial evidence checklist for UK visas