In general, most landlords require their tenants to obtain written consent prior to subleasing. Often, lease agreements include clauses regarding subleasing, so tenants should refer to their original contract before entering into a sublease agreement.
In addition to consulting their lease, tenants should also review state or municipal laws that govern subleases. In some jurisdictions, a tenant may be able to sublet without permission. However, asking for permission helps tenants maintain a positive relationship with their landlord.
If a tenant sublets without their landlord's permission, and their lease agreement prohibits subleasing, the landlord may notify the tenant of a lease violation. In this case, it's possible that the landlord would consider the subtenant an unauthorized occupant and may begin taking steps towards eviction.