diff --git a/DESCRIPTION b/DESCRIPTION
index eab6e717..96038afc 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: odin
 Title: ODE Generation and Integration
-Version: 1.1.6
+Version: 1.1.7
 Authors@R: c(person("Rich", "FitzJohn", role = c("aut", "cre"),
                     email = "rich.fitzjohn@gmail.com"),
              person("Thibaut", "Jombart", role = "ctb"),
diff --git a/R/ir_parse.R b/R/ir_parse.R
index 8009c68a..1cb09e97 100644
--- a/R/ir_parse.R
+++ b/R/ir_parse.R
@@ -9,7 +9,8 @@ ir_parse <- function(x, options, type = NULL) {
   source <- dat$source
 
   ## Data elements:
-  config <- ir_parse_config(eqs, base, root, source, options$read_include)
+  config <- ir_parse_config(eqs, base, root, source, options$read_include,
+                            options$config_custom)
   features <- ir_parse_features(eqs, config, source)
 
   variables <- ir_parse_find_variables(eqs, features$discrete, source)
diff --git a/R/ir_parse_arrays.R b/R/ir_parse_arrays.R
index 3b66aff5..7d78dbe6 100644
--- a/R/ir_parse_arrays.R
+++ b/R/ir_parse_arrays.R
@@ -86,6 +86,7 @@ ir_parse_arrays_check_usage <- function(eqs, source) {
   is_user <- vlapply(eqs, function(x) x$type == "user")
   is_copy <- vlapply(eqs, function(x) x$type == "copy")
   is_delay <- vlapply(eqs, function(x) x$type == "delay")
+  is_config <- vlapply(eqs, function(x) x$type == "config")
   is_delay_array <- vlapply(eqs, function(x)
     x$type == "delay" && !is.null(x$lhs$index))
   name_data <- vcapply(eqs, function(x) x$lhs$name_data)
@@ -126,7 +127,7 @@ ir_parse_arrays_check_usage <- function(eqs, source) {
   }
 
   ## Then, start checking for duplicates:
-  err <- is_duplicated(names(eqs)) & !is_array & !is_inplace
+  err <- is_duplicated(names(eqs)) & !is_array & !is_inplace & !is_config
   if (any(err)) {
     ir_parse_error(
       sprintf("Duplicate entries must all be array assignments (%s)",
diff --git a/R/ir_parse_config.R b/R/ir_parse_config.R
index b482793d..c6b3d25e 100644
--- a/R/ir_parse_config.R
+++ b/R/ir_parse_config.R
@@ -1,15 +1,21 @@
-ir_parse_config <- function(eqs, base_default, root, source, read_include) {
+ir_parse_config <- function(eqs, base_default, root, source,
+                            read_include, custom) {
   i <- vcapply(eqs, "[[", "type") == "config"
 
-  config <- lapply(unname(eqs[i]), ir_parse_config1, source)
+  config <- lapply(unname(eqs[i]), ir_parse_config1, source, custom)
 
   nms <- vcapply(config, function(x) x$lhs$name_data)
 
   base <- ir_parse_config_base(config[nms == "base"], base_default, source)
   include <- ir_parse_config_include(config[nms == "include"], root, source,
                                      read_include)
+  custom <- ir_parse_config_custom(config[nms %in% custom], source)
 
-  list(base = base, include = include)
+  ret <- list(base = base, include = include)
+  if (length(custom) > 0) {
+    ret$custom <- custom
+  }
+  ret
 }
 
 
@@ -71,7 +77,22 @@ ir_parse_config_include <- function(include, root, source, read_include) {
 }
 
 
-ir_parse_config1 <- function(eq, source) {
+ir_parse_config_custom <- function(x, source) {
+  if (length(x) == 0) {
+    return(NULL)
+  }
+
+  ## Is there any other validation that can really be done? We could
+  ## require that custom cases conform to particular types or are
+  ## unique? For now we'll be really leniant since we don't document
+  ## this as a public interface yet.
+  name <- vcapply(x, function(el) el$lhs$name_lhs)
+  value <- lapply(x, function(el) el$rhs$value)
+  unname(Map(list, name = name, value = value))
+}
+
+
+ir_parse_config1 <- function(eq, source, custom) {
   target <- eq$lhs$name_data
   value <- eq$rhs$value
 
@@ -79,19 +100,20 @@ ir_parse_config1 <- function(eq, source) {
     target,
     base = "character",
     include = "character",
-    ir_parse_error(sprintf("Unknown configuration option: %s", target),
-                   eq$source, source))
+    NULL)
 
-  if (!is.atomic(value)) {
-    ir_parse_error("config() rhs must be atomic (not an expression or symbol)",
-                   eq$source, source)
-  }
-
-  if (storage.mode(value) != expected_type) {
-    ir_parse_error(sprintf(
-      "Expected a %s for config(%s) but recieved a %s",
-      expected_type, target, storage.mode(value)),
-      eq$source, source)
+  if (is.null(expected_type)) {
+    if (!(target %in% custom)) {
+      ir_parse_error(sprintf("Unknown configuration option: %s", target),
+                     eq$source, source)
+    }
+  } else {
+    if (storage.mode(value) != expected_type) {
+      ir_parse_error(sprintf(
+        "Expected a %s for config(%s) but recieved a %s",
+        expected_type, target, storage.mode(value)),
+        eq$source, source)
+    }
   }
 
   eq
diff --git a/R/ir_serialise.R b/R/ir_serialise.R
index 37b3adbb..67bf75b7 100644
--- a/R/ir_serialise.R
+++ b/R/ir_serialise.R
@@ -19,7 +19,16 @@ ir_serialise_version <- function(version) {
 
 
 ir_serialise_config <- function(config) {
-  list(base = scalar(config$base), include = config$include)
+  custom <- config$custom
+  if (!is.null(config$custom)) {
+    for (i in seq_along(custom)) {
+      custom[[i]]$name <- scalar(custom[[i]]$name)
+      custom[[i]]$value <- scalar(custom[[i]]$value)
+    }
+  }
+  list(base = scalar(config$base),
+       include = config$include,
+       custom = custom)
 }
 
 
diff --git a/inst/schema.json b/inst/schema.json
index 454ca369..1debc534 100644
--- a/inst/schema.json
+++ b/inst/schema.json
@@ -151,9 +151,11 @@
                         {"$ref": "#/definitions/include"},
                         {"type": "null"}
                     ]
+                },
+                "custom": {
                 }
             },
-            "required": ["base", "include"],
+            "required": ["base", "include", "custom"],
             "additionalProperties": false
         },
 
diff --git a/tests/testthat/test-parse2-config.R b/tests/testthat/test-parse2-config.R
index 0c1f8267..ef0966ce 100644
--- a/tests/testthat/test-parse2-config.R
+++ b/tests/testthat/test-parse2-config.R
@@ -12,18 +12,12 @@ test_that("config() takes a symbol", {
                class = "odin_error")
 })
 
-test_that("config() rhs is atomic", {
-  expect_error(odin_parse("config(base) <- 1 + 1;"),
-               "config() rhs must be atomic (not an expression",
-               fixed = TRUE, class = "odin_error")
-})
-
 test_that("config(base)", {
   expect_error(odin_parse("config(base) <- 'foo'; config(base) <- 'foo'"),
                "Expected a single config(base) option",
                fixed = TRUE, class = "odin_error")
   expect_error(odin_parse("config(base) <- foo;"),
-               "config() rhs must be atomic",
+               "Expected a character for config(base) but recieved a symbol",
                fixed = TRUE, class = "odin_error")
   expect_error(
     odin_parse("config(base) <- 1;"),
@@ -61,3 +55,51 @@ test_that("config(include)", {
     "Duplicated function 'squarepulse' while reading includes",
     class = "odin_error")
 })
+
+
+test_that("Can include multiple files", {
+  ir <- odin_parse({
+    config(include) <- "user_fns.c"
+    config(include) <- "identity.c"
+    initial(x) <- 1
+    deriv(x) <- 1
+  })
+  dat <- ir_deserialise(ir)
+  expect_length(dat$config$include, 2)
+  expect_equal(
+    vcapply(dat$config$include$data, function(x) basename(x$filename[[1]])),
+    c("user_fns.c", "identity.c"))
+})
+
+
+test_that("extend config", {
+  options <- odin_options(target = "c")
+  options$config_custom <- "a"
+
+  ir <- odin_parse({
+    config(a) <- 1
+    initial(x) <- 1
+    deriv(x) <- 1
+  }, options = options)
+  expect_equal(ir_deserialise(ir)$config$custom,
+               list(list(name = "a", value = 1)))
+
+  ir <- odin_parse({
+    config(a) <- 1
+    config(a) <- 2
+    initial(x) <- 1
+    deriv(x) <- 1
+  }, options = options)
+  expect_equal(ir_deserialise(ir)$config$custom,
+               list(list(name = "a", value = 1),
+                    list(name = "a", value = 2)))
+
+  expect_error(
+    odin_parse({
+      config(a) <- 1
+      config(b) <- 2
+      initial(x) <- 1
+      deriv(x) <- 1
+    }, options = options),
+    "Unknown configuration option: b")
+})