diff --git a/ratelimiter_test.go b/ratelimiter_test.go
index d3f0ef6..aff4854 100644
--- a/ratelimiter_test.go
+++ b/ratelimiter_test.go
@@ -1,101 +1,191 @@
 package ratelimiter
 
 import (
+	"errors"
 	"sync"
 	"testing"
+	"testing/synctest"
 	"time"
 )
 
-func TestRateLimiter_Limit_Errors(t *testing.T) {
-	type TestCase struct {
-		Name     string
-		Conf     Config
-		N        int
-		ErrCount int
-		DT       time.Duration
+func TestNew_Panics(t *testing.T) {
+	cases := []struct {
+		name string
+		conf Config
+	}{
+		{"negative BurstLimit", Config{BurstLimit: -1, FillPeriod: time.Second, MaxWaitCount: 1}},
+		{"zero FillPeriod", Config{BurstLimit: 1, FillPeriod: 0, MaxWaitCount: 1}},
+		{"negative FillPeriod", Config{BurstLimit: 1, FillPeriod: -time.Second, MaxWaitCount: 1}},
+		{"negative MaxWaitCount", Config{BurstLimit: 1, FillPeriod: time.Second, MaxWaitCount: -1}},
+		{"zero BurstLimit and MaxWaitCount", Config{BurstLimit: 0, FillPeriod: time.Second, MaxWaitCount: 0}},
 	}
-
-	cases := []TestCase{
-		{
-			Name: "no burst, no wait",
-			Conf: Config{
-				BurstLimit:   0,
-				FillPeriod:   100 * time.Millisecond,
-				MaxWaitCount: 0,
-			},
-			N:        32,
-			ErrCount: 32,
-			DT:       0,
-		}, {
-			Name: "no wait",
-			Conf: Config{
-				BurstLimit:   10,
-				FillPeriod:   100 * time.Millisecond,
-				MaxWaitCount: 0,
-			},
-			N:        32,
-			ErrCount: 22,
-			DT:       0,
-		}, {
-			Name: "no burst",
-			Conf: Config{
-				BurstLimit:   0,
-				FillPeriod:   10 * time.Millisecond,
-				MaxWaitCount: 10,
-			},
-			N:        32,
-			ErrCount: 22,
-			DT:       100 * time.Millisecond,
-		}, {
-			Name: "burst and wait",
-			Conf: Config{
-				BurstLimit:   10,
-				FillPeriod:   10 * time.Millisecond,
-				MaxWaitCount: 10,
-			},
-			N:        32,
-			ErrCount: 12,
-			DT:       100 * time.Millisecond,
-		},
-	}
-
 	for _, tc := range cases {
-		wg := sync.WaitGroup{}
-		l := New(tc.Conf)
-		errs := make([]error, tc.N)
-
-		t0 := time.Now()
-
-		for i := 0; i < tc.N; i++ {
-			wg.Add(1)
-			go func(i int) {
-				errs[i] = l.Limit()
-				wg.Done()
-			}(i)
-		}
-
-		wg.Wait()
-
-		dt := time.Since(t0)
-
-		errCount := 0
-		for _, err := range errs {
-			if err != nil {
-				errCount++
-			}
-		}
-
-		if errCount != tc.ErrCount {
-			t.Fatalf("%s: Expected %d errors but got %d.",
-				tc.Name, tc.ErrCount, errCount)
-		}
-
-		if dt < tc.DT {
-			t.Fatal(tc.Name, dt, tc.DT)
-		}
-
-		if dt > tc.DT+10*time.Millisecond {
-			t.Fatal(tc.Name, dt, tc.DT)
-		}
+		t.Run(tc.name, func(t *testing.T) {
+			defer func() {
+				if recover() == nil {
+					t.Error("expected panic, got none")
+				}
+			}()
+			New(tc.conf)
+		})
 	}
 }
+
+// limitN runs n goroutines concurrently, each calling l.Limit(), and returns
+// their errors in an unordered slice.
+func limitN(l *Limiter, n int) []error {
+	errs := make([]error, n)
+	var wg sync.WaitGroup
+	for i := range n {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			errs[i] = l.Limit()
+		}(i)
+	}
+	wg.Wait()
+	return errs
+}
+
+func backoffCount(errs []error) int {
+	n := 0
+	for _, err := range errs {
+		if errors.Is(err, ErrBackoff) {
+			n++
+		}
+	}
+	return n
+}
+
+// TestLimit_Burst: burst capacity is consumed by the first BurstLimit requests;
+// excess requests are rejected without any waiting.
+func TestLimit_Burst(t *testing.T) {
+	const burst, total = 5, 10
+	synctest.Test(t, func(t *testing.T) {
+		t0 := time.Now()
+		l := New(Config{BurstLimit: burst, FillPeriod: time.Second, MaxWaitCount: 0})
+		errs := limitN(l, total)
+		if got := backoffCount(errs); got != total-burst {
+			t.Errorf("backoffs: want %d, got %d", total-burst, got)
+		}
+		if elapsed := time.Since(t0); elapsed != 0 {
+			t.Errorf("elapsed: want 0, got %v", elapsed)
+		}
+	})
+}
+
+// TestLimit_Queue: up to MaxWaitCount requests queue and sleep; excess are rejected.
+func TestLimit_Queue(t *testing.T) {
+	const maxWait, total = 5, 8
+	const period = time.Second
+	synctest.Test(t, func(t *testing.T) {
+		t0 := time.Now()
+		l := New(Config{BurstLimit: 0, FillPeriod: period, MaxWaitCount: maxWait})
+		errs := limitN(l, total)
+		if got := backoffCount(errs); got != total-maxWait {
+			t.Errorf("backoffs: want %d, got %d", total-maxWait, got)
+		}
+		// The last queued request sleeps maxWait*period.
+		if elapsed := time.Since(t0); elapsed != time.Duration(maxWait)*period {
+			t.Errorf("elapsed: want %v, got %v", time.Duration(maxWait)*period, elapsed)
+		}
+	})
+}
+
+// TestLimit_BurstAndQueue: burst requests pass immediately; overflow queues;
+// excess are rejected.
+func TestLimit_BurstAndQueue(t *testing.T) {
+	const burst, maxWait, total = 3, 5, 12
+	const period = time.Second
+	synctest.Test(t, func(t *testing.T) {
+		t0 := time.Now()
+		l := New(Config{BurstLimit: burst, FillPeriod: period, MaxWaitCount: maxWait})
+		errs := limitN(l, total)
+		if got := backoffCount(errs); got != total-(burst+maxWait) {
+			t.Errorf("backoffs: want %d, got %d", total-(burst+maxWait), got)
+		}
+		// Burst passes at T=0; the last queued request sleeps maxWait*period.
+		if elapsed := time.Since(t0); elapsed != time.Duration(maxWait)*period {
+			t.Errorf("elapsed: want %v, got %v", time.Duration(maxWait)*period, elapsed)
+		}
+	})
+}
+
+// TestLimit_Refill: capacity refills as time passes, allowing further requests.
+func TestLimit_Refill(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		l := New(Config{BurstLimit: 2, FillPeriod: time.Second, MaxWaitCount: 0})
+
+		// Exhaust burst (waitTime: -2s → -1s → 0s).
+		if err := l.Limit(); err != nil {
+			t.Fatal("call 1:", err)
+		}
+		if err := l.Limit(); err != nil {
+			t.Fatal("call 2:", err)
+		}
+		// Next would push waitTime to 1s > 0 = maxWaitTime.
+		if err := l.Limit(); !errors.Is(err, ErrBackoff) {
+			t.Fatalf("call 3: want ErrBackoff, got %v", err)
+		}
+
+		// After 2s, two tokens have refilled (waitTime: 0s - 2s = -2s, clamped to -2s).
+		time.Sleep(2 * time.Second)
+
+		// Burst is fully restored; same pattern repeats.
+		if err := l.Limit(); err != nil {
+			t.Fatal("call 4:", err)
+		}
+		if err := l.Limit(); err != nil {
+			t.Fatal("call 5:", err)
+		}
+		if err := l.Limit(); !errors.Is(err, ErrBackoff) {
+			t.Fatalf("call 6: want ErrBackoff, got %v", err)
+		}
+	})
+}
+
+// TestLimit_BurstCap: idle time does not accumulate credit beyond BurstLimit.
+//
+// After sleeping far longer than BurstLimit*FillPeriod, the limiter clamps
+// waitTime back to minWaitTime, so only BurstLimit requests can burst through.
+// Without the clamp, 10s of idle time at FillPeriod=1s would allow 10 requests.
+func TestLimit_BurstCap(t *testing.T) {
+	const burst = 2
+	synctest.Test(t, func(t *testing.T) {
+		l := New(Config{BurstLimit: burst, FillPeriod: time.Second, MaxWaitCount: 0})
+
+		time.Sleep(10 * time.Second)
+
+		errs := limitN(l, burst+3)
+		if got := backoffCount(errs); got != 3 {
+			t.Errorf("backoffs: want 3, got %d", got)
+		}
+	})
+}
+
+// TestLimitMultiple: multiple tokens are consumed per call.
+//
+// Config: BurstLimit=4, FillPeriod=1s, MaxWaitCount=2 → minWaitTime=-4s, maxWaitTime=2s.
+//
+//	Call 1: LimitMultiple(3) → waitTime = -4s+3s = -1s  → immediate
+//	Call 2: LimitMultiple(3) → waitTime = -1s+3s =  2s  → sleeps 2s
+//	Call 3: LimitMultiple(3) → waitTime =  2s+3s =  5s  > 2s → ErrBackoff
+func TestLimitMultiple(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		t0 := time.Now()
+		l := New(Config{BurstLimit: 4, FillPeriod: time.Second, MaxWaitCount: 2})
+
+		if err := l.LimitMultiple(3); err != nil {
+			t.Fatalf("call 1: %v", err)
+		}
+		if err := l.LimitMultiple(3); err != nil {
+			t.Fatalf("call 2: %v", err)
+		}
+		if err := l.LimitMultiple(3); !errors.Is(err, ErrBackoff) {
+			t.Fatalf("call 3: want ErrBackoff, got %v", err)
+		}
+		if elapsed := time.Since(t0); elapsed != 2*time.Second {
+			t.Errorf("elapsed: want 2s, got %v", elapsed)
+		}
+	})
+}