diff --git a/api.go b/api.go
index 7631138..88144fc 100644
--- a/api.go
+++ b/api.go
@@ -26,10 +26,11 @@ type programTree struct {
 	CommandFn       CommandFn
 	HelpCommandName string
 	mode            Mode
-	unknownMode     UnknownMode // Unknown option mode
-	requireOrder    bool        // stop parsing args as soon as an unknown is found
-	skipOptionsCopy bool        // skips copying options from parent to child. Required when doing wrapper commands.
-	Suggestions     []string    // Suggestions used for completions
+	unknownMode     UnknownMode    // Unknown option mode
+	requireOrder    bool           // stop parsing args as soon as an unknown is found
+	skipOptionsCopy bool           // skips copying options from parent to child. Required when doing wrapper commands.
+	Suggestions     []string       // Suggestions used for completions
+	SuggestionFns   []CompletionFn // SuggestionsFns used for completions
 
 	mapKeysToLower bool // controls wether or not map keys are normalized to lowercase
 
@@ -275,6 +276,12 @@ ARGS_LOOP:
 					}
 				}
 			}
+			// SuggestionFns
+			{
+				for _, fn := range currentProgramNode.SuggestionFns {
+					completions = append(completions, fn(completionMode, iterator.Value())...)
+				}
+			}
 
 			// Provide other kinds of completions, like file completions.
 
diff --git a/changelog.adoc b/changelog.adoc
index 7a0a649..59efd64 100644
--- a/changelog.adoc
+++ b/changelog.adoc
@@ -1,6 +1,17 @@
 = Changelog
 :toc:
 
+== wip v0.30.0: New Features
+
+As the releases before, this release has 100% test coverage.
+Tested with Go 1.16, 1.17, 1.18, 1.19, 1.20 and 1.21.
+
+=== New Features
+
+* add `opt.CustomCompletionFn` which allows to provide a custom completion function that gets lazily called.
++
+The completion function receives the target shell (bash or zsh) and the current partial completion.
+
 == v0.29.0: New Features
 
 As the releases before, this release has 100% test coverage.
diff --git a/examples/complex/complete/complete.go b/examples/complex/complete/complete.go
new file mode 100644
index 0000000..88b67d2
--- /dev/null
+++ b/examples/complex/complete/complete.go
@@ -0,0 +1,41 @@
+package complete
+
+import (
+	"context"
+	"io"
+	"log"
+	"strings"
+
+	"github.com/DavidGamba/go-getoptions"
+)
+
+var Logger = log.New(io.Discard, "log ", log.LstdFlags)
+
+// NewCommand - Populate Options definition
+func NewCommand(parent *getoptions.GetOpt) *getoptions.GetOpt {
+	opt := parent.NewCommand("complete", "Example completions")
+	opt.SetCommandFn(Run)
+	opt.CustomCompletion("dev-east", "dev-west", "staging-east", "prod-east", "prod-west", "prod-south")
+	opt.CustomCompletionFn(func(target, s string) []string {
+		if strings.HasPrefix("dev-", s) {
+			return []string{"dev-hola/", "dev-hello"}
+		}
+		if strings.HasPrefix("dev-h", s) {
+			return []string{"dev-hola/", "dev-hello"}
+		}
+		if strings.HasPrefix("dev-hello", s) {
+			return []string{"dev-hello"}
+		}
+		if strings.HasPrefix("dev-hola/", s) {
+			return []string{"dev-hola/a", "dev-hola/b", "dev-hola/" + target}
+		}
+		return []string{}
+	})
+	return opt
+}
+
+// Run - Command entry point
+func Run(ctx context.Context, opt *getoptions.GetOpt, args []string) error {
+	Logger.Printf("args: %v\n", args)
+	return nil
+}
diff --git a/examples/complex/main.go b/examples/complex/main.go
index ac412ff..f03d11b 100644
--- a/examples/complex/main.go
+++ b/examples/complex/main.go
@@ -7,12 +7,14 @@ import (
 	"log"
 	"os"
 
-	"github.com/DavidGamba/go-getoptions"
+	complexcomplete "github.com/DavidGamba/go-getoptions/examples/complex/complete"
 	complexgreet "github.com/DavidGamba/go-getoptions/examples/complex/greet"
 	complexlog "github.com/DavidGamba/go-getoptions/examples/complex/log"
 	complexlswrapper "github.com/DavidGamba/go-getoptions/examples/complex/lswrapper"
 	complexshow "github.com/DavidGamba/go-getoptions/examples/complex/show"
 	complexslow "github.com/DavidGamba/go-getoptions/examples/complex/slow"
+
+	"github.com/DavidGamba/go-getoptions"
 )
 
 var Logger = log.New(io.Discard, "", log.LstdFlags)
@@ -34,6 +36,7 @@ func program(args []string) int {
 	complexlswrapper.NewCommand(opt)
 	complexshow.NewCommand(opt)
 	complexslow.NewCommand(opt)
+	complexcomplete.NewCommand(opt)
 	opt.HelpCommand("help", opt.Alias("?"))
 	remaining, err := opt.Parse(args[1:])
 	if err != nil {
@@ -47,6 +50,7 @@ func program(args []string) int {
 		complexlswrapper.Logger.SetOutput(os.Stderr)
 		complexshow.Logger.SetOutput(os.Stderr)
 		complexslow.Logger.SetOutput(os.Stderr)
+		complexcomplete.Logger.SetOutput(os.Stderr)
 	}
 	if opt.Called("profile") {
 		Logger.Printf("profile: %s\n", opt.Value("profile"))
diff --git a/user.go b/user.go
index 2f3e6ac..dad88e1 100644
--- a/user.go
+++ b/user.go
@@ -187,6 +187,22 @@ func (gopt *GetOpt) CustomCompletion(list ...string) *GetOpt {
 	return gopt
 }
 
+// CompletionFn - Function receiver for custom completions.
+// The `target` argument indicates "bash" or "zsh" for the completion targets.
+//
+// NOTE: Bash completions have = as a special char and results should be trimmed from the = on.
+// This should not be done for Zsh.
+type CompletionFn func(target, partialCompletion string) []string
+
+// CustomCompletionFn - Allows to define custom completion functions that get lazily called.
+//
+// NOTE: Bash completions have = as a special char and results should be trimmed from the = on.
+// This should not be done for Zsh.
+func (gopt *GetOpt) CustomCompletionFn(fn ...CompletionFn) *GetOpt {
+	gopt.programTree.SuggestionFns = append(gopt.programTree.SuggestionFns, fn...)
+	return gopt
+}
+
 // UnsetOptions - Unsets inherited options from parent program and parent commands.
 // This is useful when writing wrappers around other commands.
 //