diff --git a/R/ParamSet.R b/R/ParamSet.R
index 1aeb3087..16275b13 100644
--- a/R/ParamSet.R
+++ b/R/ParamSet.R
@@ -379,7 +379,9 @@ ParamSet = R6Class("ParamSet",
         return(private$.values)
       }
 
+      xs = Reduce(function(val, fun) fun(val), self$callbacks, xs)
       self$assert(xs)
+
       if (length(xs) == 0L) xs = named_list()
       private$.values = xs
     },
@@ -390,6 +392,14 @@ ParamSet = R6Class("ParamSet",
 
     extra_values = function() {
       private$.values[names(private$.values) %nin% names(private$.params)]
+    },
+
+    callbacks = function(val) {
+      if (!missing(val)) {
+        assert_list(val, types = "function", any.missing = FALSE)
+        private$.callbacks = val
+      }
+      private$.callbacks
     }
   ),
 
@@ -399,6 +409,7 @@ ParamSet = R6Class("ParamSet",
     .params = NULL,
     .values = named_list(),
     .deps = data.table(id = character(0L), on = character(0L), cond = list()),
+    .callbacks = list(),
     # return a slot / AB, as a named vec, named with id (and can enforce a certain vec-type)
     get_member_with_idnames = function(member, astype) set_names(astype(map(self$params, member)), names(self$params)),
 
diff --git a/R/ParamSetCollection.R b/R/ParamSetCollection.R
index fe163466..2f7846bd 100644
--- a/R/ParamSetCollection.R
+++ b/R/ParamSetCollection.R
@@ -115,13 +115,11 @@ ParamSetCollection = R6Class("ParamSetCollection", inherit = ParamSet,
     },
 
     values = function(xs) {
-      sets = private$.sets
-      names(sets) = map_chr(sets, "set_id")
       if (!missing(xs)) {
-        assert_list(xs)
+        xs = Reduce(function(val, fun) fun(val), self$callbacks, xs)
         self$assert(xs) # make sure everything is valid and feasible
 
-        for (s in sets) {
+        for (s in private$.sets) {
           # retrieve sublist for each set, then assign it in set (after removing prefix)
           psids = names(s$params)
           if (s$set_id != "") {
@@ -134,6 +132,8 @@ ParamSetCollection = R6Class("ParamSetCollection", inherit = ParamSet,
           s$values = pv
         }
       }
+      sets = private$.sets
+      names(sets) = map_chr(sets, "set_id")
       vals = map(sets, "values")
       vals = unlist(vals, recursive = FALSE)
       if (!length(vals)) vals = named_list()
diff --git a/tests/testthat/test_ParamSet.R b/tests/testthat/test_ParamSet.R
index 1956f01c..55b7146b 100644
--- a/tests/testthat/test_ParamSet.R
+++ b/tests/testthat/test_ParamSet.R
@@ -274,3 +274,130 @@ test_that("required tag, empty param set (#219)", {
   ps$ids()
   expect_identical(ps$ids(tags = "required"), character(0))
 })
+
+test_that("callbacks", {
+  ps = ParamSet$new(list(
+    ParamDbl$new(id = "x", lower = 1, tags = c("t1")),
+    ParamInt$new(id = "y", lower = 1, upper = 2),
+    ParamFct$new(id = "z", levels = letters[1:3], tags = c("t1"))
+  ))
+  ps$callbacks[[1]] = function(x) {
+    x$x = 2
+    x
+  }
+  expect_equal(ps$values, named_list())
+  ps$values$y = 1
+  expect_equal(ps$values, list(y = 1, x = 2))
+  ps$values$x = 1
+  expect_equal(ps$values, list(y = 1, x = 2))
+  ps$callbacks[[2]] = function(x) {
+    x$x = 1
+    x
+  }
+  ps$values$y = 1
+  expect_equal(ps$values, list(y = 1, x = 1))
+  ps$callbacks[[2]] = function(x) {
+    x$x = 0
+    x
+  }
+  expect_error({ps$values = list(y = 1)}, "is not >= 1")
+
+  ps$callbacks[[2]] = function(x) {
+    x$x = 1
+    x
+  }
+  ps$callbacks[[1]] = function(x) {
+    x$x = 0
+    x
+  }
+  ps$values = list(y = 2)
+  expect_equal(ps$values, list(y = 2, x = 1))
+  ps$callbacks[[1]] = NULL
+  ps$values = list(y = 1, x = 2)
+  expect_equal(ps$values, list(y = 1, x = 1))
+})
+
+test_that("callbacks on ParamSetCollection", {
+
+  psetset = function() {
+    ps = ParamSet$new(list(ParamUty$new("paramset", custom_check = function(x) check_class(x, "ParamSet", null.ok = TRUE))))
+    psc = ParamSetCollection$new(list(ps))
+
+    psc$callbacks[[1]] = function(x) {
+      prevset = psc$values$paramset
+      newset = x$paramset
+      if (!identical(x$paramset, prevset)) {
+        psc$params$paramset$assert(newset)
+        if (!is.null(newset)) {
+          xcpy = x
+          xcpy$paramset = NULL
+          newset$assert(xcpy)
+        } else {
+          ParamSet$new()$assert(x)
+        }
+        psc$remove_sets("")
+        psc$add(ps)
+        if (!is.null(newset)) {
+          psc$add(newset)
+        }
+      }
+      x
+    }
+    psc
+  }
+
+  ps = psetset()
+
+  ps1 = ParamSet$new(list(
+    ParamDbl$new(id = "x", lower = 1, tags = c("t1")),
+    ParamInt$new(id = "y", lower = 1, upper = 2),
+    ParamFct$new(id = "z", levels = letters[1:3], tags = c("t1"))
+  ))
+
+  ps2 = ParamDbl$new("a")$rep(3)
+
+  expect_equal(names(ps$params), "paramset")
+
+  ps$values$paramset = ps1
+
+  expect_equal(names(ps$params), c("paramset", "x", "y", "z"))
+
+  ps$values$x = 1
+  expect_equal(ps$values, list(paramset = ps1, x = 1))
+
+  ps$values = list(paramset = ps2, a_rep_1 = 0)
+
+  expect_equal(ps$values, list(paramset = ps2, a_rep_1 = 0))
+
+  # The problem here is that there is an ambiguity. suppose
+  # > psB$values = list(x = 2)
+  # > ps$values = list(paramset = psA, x = 1)
+  # Now the command
+  # (A) > ps$values = list(paramset = psB, x = 1)
+  # and the command
+  # (B) > ps$values$paramset = psB
+  # are functionally the same, but in case (B) we wished we could
+  # keep the parameter values of psB. However, because (A) is done
+  # by things like tuning, it takes precedent and must work as
+  # expected. Therefore the following throws an error.
+  expect_error({ps$values$paramset = ps1}, "a_rep_1.* not available")
+
+  expect_equal(ps$values, list(paramset = ps2, a_rep_1 = 0))
+
+  ps$values = c(list(paramset = ps1), ps1$values)
+
+  expect_equal(ps$values, list(paramset = ps1, x = 1))
+
+  expect_error({ps$values = list(x = 2)}, "Parameter 'x' not available")
+
+  expect_equal(ps$values, list(paramset = ps1, x = 1))
+
+  expect_error({ps$values$paramset = NULL}, "Parameter 'x' not available")
+
+  expect_equal(ps$values, list(paramset = ps1, x = 1))
+
+  ps$values = list()
+
+  expect_equal(ps$values, named_list())
+
+})