What is IMSS?
The IMSS (Instituto Mexicano del Seguro Social, the Mexican Social Security Institute) administers social security in Mexico: sickness, maternity, disability, work risks, retirement and childcare. Every employee must be registered.
Contributions are split between the cuota obrera (the employee share, which the employer withholds from salary) and the cuota patronal (the employer share, paid by the employer). The base is the Salario Base de Cotización (SBC, the contribution base salary).
In practice
From a monthly salary, the employer withholds the employee's IMSS share (approximately 2.375%) and pays it to the Institute together with its own employer contribution.