diff --git a/Gopkg.lock b/Gopkg.lock
index 907ab5fca0..1461563fde 100644
--- a/Gopkg.lock
+++ b/Gopkg.lock
@@ -537,6 +537,7 @@
     "fileutil",
     "index",
     "labels",
+    "testutil",
     "wal",
   ]
   pruneopts = ""
@@ -853,6 +854,7 @@
     "github.com/prometheus/common/version",
     "github.com/prometheus/prometheus/discovery/file",
     "github.com/prometheus/prometheus/discovery/targetgroup",
+    "github.com/prometheus/prometheus/pkg/gate",
     "github.com/prometheus/prometheus/pkg/labels",
     "github.com/prometheus/prometheus/pkg/timestamp",
     "github.com/prometheus/prometheus/pkg/value",
@@ -867,6 +869,7 @@
     "github.com/prometheus/tsdb/fileutil",
     "github.com/prometheus/tsdb/index",
     "github.com/prometheus/tsdb/labels",
+    "github.com/prometheus/tsdb/testutil",
     "golang.org/x/net/context",
     "golang.org/x/sync/errgroup",
     "golang.org/x/text/language",
diff --git a/cmd/thanos/store.go b/cmd/thanos/store.go
index ddf9893061..25f8512abf 100644
--- a/cmd/thanos/store.go
+++ b/cmd/thanos/store.go
@@ -36,6 +36,11 @@ func registerStore(m map[string]setupFunc, app *kingpin.Application, name string
 	chunkPoolSize := cmd.Flag("chunk-pool-size", "Maximum size of concurrently allocatable bytes for chunks.").
 		Default("2GB").Bytes()
 
+	maxSampleCount := cmd.Flag("grpc-sample-limit", "Maximum amount of samples returned via a single Series call. 0 means no limit.").
+		Default("50000000").Uint()
+
+	maxConcurrent := cmd.Flag("grpc-concurrent-limit", "Maximum number of concurrent Series calls. 0 means no limit.").Default("20").Int()
+
 	objStoreConfig := regCommonObjStoreFlags(cmd, "", true)
 
 	syncInterval := cmd.Flag("sync-block-duration", "Repeat interval for syncing the blocks between local and remote view.").
@@ -63,6 +68,8 @@ func registerStore(m map[string]setupFunc, app *kingpin.Application, name string
 			peer,
 			uint64(*indexCacheSize),
 			uint64(*chunkPoolSize),
+			uint64(*maxSampleCount),
+			int(*maxConcurrent),
 			name,
 			debugLogging,
 			*syncInterval,
@@ -87,6 +94,8 @@ func runStore(
 	peer cluster.Peer,
 	indexCacheSizeBytes uint64,
 	chunkPoolSizeBytes uint64,
+	maxSampleCount uint64,
+	maxConcurrent int,
 	component string,
 	verbose bool,
 	syncInterval time.Duration,
@@ -117,6 +126,8 @@ func runStore(
 			dataDir,
 			indexCacheSizeBytes,
 			chunkPoolSizeBytes,
+			maxSampleCount,
+			maxConcurrent,
 			verbose,
 			blockSyncConcurrency,
 		)
diff --git a/docs/components/store.md b/docs/components/store.md
index e32aa11d16..866cfe2642 100644
--- a/docs/components/store.md
+++ b/docs/components/store.md
@@ -104,6 +104,12 @@ Flags:
       --index-cache-size=250MB   Maximum size of items held in the index cache.
       --chunk-pool-size=2GB      Maximum size of concurrently allocatable bytes
                                  for chunks.
+      --grpc-sample-limit=50000000  
+                                 Maximum amount of samples returned via a single
+                                 Series call. 0 means no limit.
+      --grpc-concurrent-limit=20  
+                                 Maximum number of concurrent Series calls. 0
+                                 means no limit.
       --objstore.config-file=<bucket.config-yaml-path>  
                                  Path to YAML file that contains object store
                                  configuration.
diff --git a/pkg/compact/downsample/downsample_test.go b/pkg/compact/downsample/downsample_test.go
index bb2c38b17a..2a56426c96 100644
--- a/pkg/compact/downsample/downsample_test.go
+++ b/pkg/compact/downsample/downsample_test.go
@@ -1,13 +1,14 @@
 package downsample
 
 import (
-	"github.com/prometheus/tsdb"
 	"io/ioutil"
 	"math"
 	"os"
 	"path/filepath"
 	"testing"
 
+	"github.com/prometheus/tsdb"
+
 	"github.com/improbable-eng/thanos/pkg/block/metadata"
 
 	"github.com/prometheus/prometheus/pkg/value"
diff --git a/pkg/store/bucket.go b/pkg/store/bucket.go
index e006f5c70c..090d4367c5 100644
--- a/pkg/store/bucket.go
+++ b/pkg/store/bucket.go
@@ -171,6 +171,13 @@ type BucketStore struct {
 	debugLogging bool
 	// Number of goroutines to use when syncing blocks from object storage.
 	blockSyncConcurrency int
+
+	// The maximum of samples Thanos Store could return in one Series() call.
+	// Set to 0 to remove this limit (not recommended).
+	maxSampleCount uint64
+
+	// Query gate which limits the maximum amount of concurrent queries.
+	queryGate *Gate
 }
 
 // NewBucketStore creates a new bucket backed store that implements the store API against
@@ -182,6 +189,8 @@ func NewBucketStore(
 	dir string,
 	indexCacheSizeBytes uint64,
 	maxChunkPoolBytes uint64,
+	maxSampleCount uint64,
+	maxConcurrent int,
 	debugLogging bool,
 	blockSyncConcurrency int,
 ) (*BucketStore, error) {
@@ -204,8 +213,10 @@ func NewBucketStore(
 		chunkPool:            chunkPool,
 		blocks:               map[ulid.ULID]*bucketBlock{},
 		blockSets:            map[uint64]*bucketBlockSet{},
+		maxSampleCount:       maxSampleCount,
 		debugLogging:         debugLogging,
 		blockSyncConcurrency: blockSyncConcurrency,
+		queryGate:            NewGate(maxConcurrent, reg),
 	}
 	s.metrics = newBucketStoreMetrics(reg)
 
@@ -463,7 +474,7 @@ func (s *bucketSeriesSet) Err() error {
 	return s.err
 }
 
-func (s *BucketStore) blockSeries(
+func (bs *BucketStore) blockSeries(
 	ctx context.Context,
 	ulid ulid.ULID,
 	extLset map[string]string,
@@ -471,6 +482,8 @@ func (s *BucketStore) blockSeries(
 	chunkr *bucketChunkReader,
 	matchers []labels.Matcher,
 	req *storepb.SeriesRequest,
+	samples *uint64,
+	samplesLock *sync.Mutex,
 ) (storepb.SeriesSet, *queryStats, error) {
 	ps, err := indexr.ExpandedPostings(matchers)
 	if err != nil {
@@ -559,7 +572,7 @@ func (s *BucketStore) blockSeries(
 			if err != nil {
 				return nil, nil, errors.Wrap(err, "get chunk")
 			}
-			if err := populateChunk(&s.chks[i], chk, req.Aggregates); err != nil {
+			if err := bs.populateChunk(&s.chks[i], chk, req.Aggregates, samples, samplesLock); err != nil {
 				return nil, nil, errors.Wrap(err, "populate chunk")
 			}
 		}
@@ -568,7 +581,9 @@ func (s *BucketStore) blockSeries(
 	return newBucketSeriesSet(res), indexr.stats.merge(chunkr.stats), nil
 }
 
-func populateChunk(out *storepb.AggrChunk, in chunkenc.Chunk, aggrs []storepb.Aggr) error {
+func (bs *BucketStore) populateChunk(out *storepb.AggrChunk, in chunkenc.Chunk, aggrs []storepb.Aggr,
+	samples *uint64, samplesLock *sync.Mutex) error {
+
 	if in.Encoding() == chunkenc.EncXOR {
 		out.Raw = &storepb.Chunk{Type: storepb.Chunk_XOR, Data: in.Bytes()}
 		return nil
@@ -579,6 +594,14 @@ func populateChunk(out *storepb.AggrChunk, in chunkenc.Chunk, aggrs []storepb.Ag
 
 	ac := downsample.AggrChunk(in.Bytes())
 
+	samplesLock.Lock()
+	*samples += uint64(ac.NumSamples())
+	if bs.maxSampleCount > 0 && *samples > bs.maxSampleCount {
+		samplesLock.Unlock()
+		return errors.Errorf("sample limit violated (got %v, limit %v)", *samples, bs.maxSampleCount)
+	}
+	samplesLock.Unlock()
+
 	for _, at := range aggrs {
 		switch at {
 		case storepb.Aggr_COUNT:
@@ -652,19 +675,24 @@ func debugFoundBlockSetOverview(logger log.Logger, mint, maxt int64, lset labels
 }
 
 // Series implements the storepb.StoreServer interface.
-// TODO(bwplotka): It buffers all chunks in memory and only then streams to client.
-// 1. Either count chunk sizes and error out too big query.
-// 2. Stream posting -> series -> chunk all together.
 func (s *BucketStore) Series(req *storepb.SeriesRequest, srv storepb.Store_SeriesServer) error {
+	err := s.queryGate.Start(srv.Context())
+	if err != nil {
+		return errors.Wrapf(err, "gate Start failed")
+	}
+	defer s.queryGate.Done()
+
 	matchers, err := translateMatchers(req.Matchers)
 	if err != nil {
 		return status.Error(codes.InvalidArgument, err.Error())
 	}
 	var (
-		stats = &queryStats{}
-		g     run.Group
-		res   []storepb.SeriesSet
-		mtx   sync.Mutex
+		stats       = &queryStats{}
+		g           run.Group
+		res         []storepb.SeriesSet
+		mtx         sync.Mutex
+		samples     uint64
+		samplesLock sync.Mutex
 	)
 	s.mtx.RLock()
 
@@ -701,6 +729,8 @@ func (s *BucketStore) Series(req *storepb.SeriesRequest, srv storepb.Store_Serie
 					chunkr,
 					blockMatchers,
 					req,
+					&samples,
+					&samplesLock,
 				)
 				if err != nil {
 					return errors.Wrapf(err, "fetch series for block %s", b.meta.ULID)
diff --git a/pkg/store/bucket_e2e_test.go b/pkg/store/bucket_e2e_test.go
index 28122ac367..0f3850dc0d 100644
--- a/pkg/store/bucket_e2e_test.go
+++ b/pkg/store/bucket_e2e_test.go
@@ -35,7 +35,7 @@ func (s *storeSuite) Close() {
 	s.wg.Wait()
 }
 
-func prepareStoreWithTestBlocks(t testing.TB, dir string, bkt objstore.Bucket) *storeSuite {
+func prepareStoreWithTestBlocks(t testing.TB, dir string, bkt objstore.Bucket, maxSampleCount uint64) *storeSuite {
 	series := []labels.Labels{
 		labels.FromStrings("a", "1", "b", "1"),
 		labels.FromStrings("a", "1", "b", "2"),
@@ -87,7 +87,7 @@ func prepareStoreWithTestBlocks(t testing.TB, dir string, bkt objstore.Bucket) *
 		testutil.Ok(t, os.RemoveAll(dir2))
 	}
 
-	store, err := NewBucketStore(log.NewLogfmtLogger(os.Stderr), nil, bkt, dir, 100, 0, false, 20)
+	store, err := NewBucketStore(log.NewLogfmtLogger(os.Stderr), nil, bkt, dir, 100, 0, maxSampleCount, 20, false, 20)
 	testutil.Ok(t, err)
 
 	s.store = store
@@ -126,7 +126,7 @@ func TestBucketStore_e2e(t *testing.T) {
 		testutil.Ok(t, err)
 		defer func() { testutil.Ok(t, os.RemoveAll(dir)) }()
 
-		s := prepareStoreWithTestBlocks(t, dir, bkt)
+		s := prepareStoreWithTestBlocks(t, dir, bkt, 0)
 		defer s.Close()
 
 		mint, maxt := s.store.TimeRange()
@@ -215,6 +215,31 @@ func TestBucketStore_e2e(t *testing.T) {
 			MaxTime: maxt,
 		}, srv))
 		testutil.Equals(t, 0, len(srv.SeriesSet))
+
+		// Test the samples limit.
+		testutil.Ok(t, os.RemoveAll(dir))
+		s = prepareStoreWithTestBlocks(t, dir, bkt, 10)
+		mint, maxt = s.store.TimeRange()
+		defer s.Close()
+
+		srv = newStoreSeriesServer(ctx)
+
+		testutil.Ok(t, s.store.Series(&storepb.SeriesRequest{
+			Matchers: []storepb.LabelMatcher{
+				{Type: storepb.LabelMatcher_EQ, Name: "a", Value: "1"},
+			},
+			MinTime: mint,
+			MaxTime: maxt,
+		}, srv))
+
+		testutil.NotOk(t, s.store.Series(&storepb.SeriesRequest{
+			Matchers: []storepb.LabelMatcher{
+				{Type: storepb.LabelMatcher_RE, Name: "a", Value: "1|2"},
+			},
+			MinTime: mint,
+			MaxTime: maxt,
+		}, srv))
+
 	})
 
 }
diff --git a/pkg/store/gate.go b/pkg/store/gate.go
new file mode 100644
index 0000000000..555aa1828f
--- /dev/null
+++ b/pkg/store/gate.go
@@ -0,0 +1,43 @@
+package store
+
+import (
+	"context"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/prometheus/pkg/gate"
+)
+
+// Gate wraps the Prometheus gate with extra metrics.
+type Gate struct {
+	g              *gate.Gate
+	currentQueries prometheus.Gauge
+}
+
+// NewGate returns a new gate.
+func NewGate(maxConcurrent int, reg prometheus.Registerer) *Gate {
+	g := &Gate{
+		g: gate.New(maxConcurrent),
+	}
+	g.currentQueries = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "thanos_bucket_store_queries_total",
+		Help: "Total number of currently executing queries.",
+	})
+
+	if reg != nil {
+		reg.MustRegister(g.currentQueries)
+	}
+
+	return g
+}
+
+// Start iniates a new query.
+func (g *Gate) Start(ctx context.Context) error {
+	g.currentQueries.Inc()
+	return g.g.Start(ctx)
+}
+
+// Done finishes a query.
+func (g *Gate) Done() {
+	g.currentQueries.Dec()
+	g.g.Done()
+}