In Kotlin, you declare a nullable variable by adding ? after the type.

var name: String? = null

This means name can hold either a String value or null.

Examples:

var age: Int? = null
age = 25

val email: String? = "[email protected]"
val phone: String? = null

Without ?, Kotlin does not allow null:

var name: String = null // Error

With ?, you must handle possible null safely:

val name: String? = null

println(name?.length) // Safe call, prints null instead of crashing

You can also provide a default value with the Elvis operator ?::

val name: String? = null
val length = name?.length ?: 0

println(length) // 0

So the basic pattern is:

var variableName: Type? = null