diff --git a/testscript/doc.go b/testscript/doc.go
index f47a7d98..8ad2272b 100644
--- a/testscript/doc.go
+++ b/testscript/doc.go
@@ -121,6 +121,33 @@ A condition can be negated: [!short] means to run the rest of the line
 when testing.Short() is false.
 
 Additional conditions can be added by passing a function to Params.Condition.
+The function will only be called when all built-in conditions have been checked for.
+
+An example:
+		Condition: func(cond string) (bool, error) {
+			// Assume condition name and args are separated by colon (":")
+			args := strings.Split(cond, ":")
+			name := args[0]
+			switch name {
+			case "exists":
+				if len(args) < 2 {
+					return false, fmt.Errorf("syntax: [exists:file_name]")
+				}
+				_, err := os.Stat(args[1])
+				return !errors.Is(err, fs.ErrNotExist), nil
+			case "long":
+				return os.Getenv("TEST_LONG") != "", nil
+			default:
+				return false, fmt.Errorf("unrecognized condition %s", name)
+			}
+		},
+
+With the conditions so defined, you can use them as follows:
+	env file_name=/path/to/filename
+	[exists:$file_name] exec echo 'file was found'
+	env loops=1
+	[long] env loops=3
+	exec do_something $loops
 
 The predefined commands are:
 
diff --git a/testscript/testdata/custom_conditions.txt b/testscript/testdata/custom_conditions.txt
new file mode 100644
index 00000000..8c1018f3
--- /dev/null
+++ b/testscript/testdata/custom_conditions.txt
@@ -0,0 +1,67 @@
+[!exec:echo] skip
+
+env upper_word=ABCD
+env lower_word=abcd
+
+[always_true] exec echo 'this is true'
+stdout 'this is true'
+
+exec echo ''
+
+[!always_true] exec echo 'this is true'
+! stdout .
+
+exec echo ''
+
+[always_false] exec echo 'this is false'
+! stdout .
+
+exec echo ''
+
+[!always_false] exec echo 'this is false'
+stdout 'this is false'
+
+exec echo ''
+
+[is_upper:ABCD] exec echo 'it is upper'
+stdout 'it is upper'
+
+exec echo ''
+
+[is_upper:$upper_word] exec echo 'it is again upper'
+stdout 'it is again upper'
+
+exec echo ''
+
+[is_upper:abcd] exec echo 'it is upper'
+! stdout .
+
+exec echo ''
+
+[is_upper:$lower_word] exec echo 'it is lower'
+! stdout .
+
+exec echo ''
+
+[!is_upper:ABCD] exec echo 'it is upper'
+! stdout .
+
+exec echo ''
+
+[is_lower:abcd] exec echo  'it is lower'
+stdout 'it is lower'
+
+exec echo ''
+
+[is_lower:$lower_word] exec echo  'it is again lower'
+stdout 'it is again lower'
+
+exec echo ''
+
+[is_lower:ABCD] exec echo 'it is lower'
+! stdout .
+
+exec echo ''
+
+[!is_lower:$lower_word] exec echo 'it is lower'
+! stdout .
diff --git a/testscript/testdata/custom_conditions_errors.txt b/testscript/testdata/custom_conditions_errors.txt
new file mode 100644
index 00000000..65301bde
--- /dev/null
+++ b/testscript/testdata/custom_conditions_errors.txt
@@ -0,0 +1,72 @@
+env GOPATH=$WORK
+env GOCACHE=$WORK/.cache
+
+[!exec:echo] skip
+cd cond_errors
+exec go mod tidy
+! exec go test -run TestConditionErrors/is_upper-no-parameter
+stdout 'FAIL'
+stdout 'syntax: \[is_upper:word\]'
+
+! exec go test -run TestConditionErrors/is_lower-no-parameter
+stdout 'FAIL'
+stdout 'syntax: \[is_lower:word\]'
+
+! exec go test -run TestConditionErrors/unrecognized
+stdout 'FAIL'
+stdout 'unrecognized condition something'
+
+-- cond_errors/main_test.go --
+package condition_errors
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/rogpeppe/go-internal/testscript"
+)
+
+func TestConditionErrors(t *testing.T) {
+	testscript.Run(t, testscript.Params{
+		Dir: "testdata",
+		Condition: func(cond string) (bool, error) {
+			// Assume condition name and args are separated by colon (":")
+			args := strings.Split(cond, ":")
+			name := args[0]
+			switch name {
+			case "is_upper":
+				if len(args) < 2 {
+					return false, fmt.Errorf("syntax: [is_upper:word]")
+				}
+				return strings.ToUpper(args[1]) == args[1], nil
+			case "is_lower":
+				if len(args) < 2 {
+					return false, fmt.Errorf("syntax: [is_lower:word]")
+				}
+				return strings.ToLower(args[1]) == args[1], nil
+			case "always_true":
+				return true, nil
+			case "always_false":
+				return false, nil
+			default:
+				return false, fmt.Errorf("unrecognized condition %s", name)
+			}
+		},
+	})
+}
+-- cond_errors/go.mod --
+module condition_errors
+
+go 1.18
+
+require (
+	github.com/rogpeppe/go-internal v1.9.0
+)
+
+-- cond_errors/testdata/is_upper-no-parameter.txt --
+[is_upper] exec echo ''
+-- cond_errors/testdata/is_lower-no-parameter.txt --
+[is_lower] exec echo ''
+-- cond_errors/testdata/unrecognized.txt --
+[something] exec echo ''
diff --git a/testscript/testscript_test.go b/testscript/testscript_test.go
index 5bf42cde..f9e4d597 100644
--- a/testscript/testscript_test.go
+++ b/testscript/testscript_test.go
@@ -254,6 +254,29 @@ func TestScripts(t *testing.T) {
 			},
 			"echoandexit": echoandexit,
 		},
+		Condition: func(cond string) (bool, error) {
+			// Assume condition name and args are separated by colon (":")
+			args := strings.Split(cond, ":")
+			name := args[0]
+			switch name {
+			case "is_upper":
+				if len(args) < 2 {
+					return false, fmt.Errorf("syntax: [is_upper:word]")
+				}
+				return strings.ToUpper(args[1]) == args[1], nil
+			case "is_lower":
+				if len(args) < 2 {
+					return false, fmt.Errorf("syntax: [is_lower:word]")
+				}
+				return strings.ToLower(args[1]) == args[1], nil
+			case "always_true":
+				return true, nil
+			case "always_false":
+				return false, nil
+			default:
+				return false, fmt.Errorf("unrecognized condition %s", name)
+			}
+		},
 		Setup: func(env *Env) error {
 			infos, err := ioutil.ReadDir(env.WorkDir)
 			if err != nil {