diff --git a/Project.toml b/Project.toml
index 55758812..d2aaf0e6 100644
--- a/Project.toml
+++ b/Project.toml
@@ -7,10 +7,17 @@ version = "1.18.1"
 Base64 = "2a0f44e3-6c83-55bd-87e4-b1978d98bd5f"
 CodecZlib = "944b1d66-785c-5afd-91f1-9de20f533193"
 FillArrays = "1a297f60-69ca-5386-bcde-b61e274b549b"
+HDF5 = "f67ccb44-e63f-5c2f-98bd-6dc0ccc4ba2f"
 LightXML = "9c8b4983-aa76-5018-a973-4c85ecc9e179"
 TranscodingStreams = "3bb67fe8-82b1-5028-8e26-92a6c54297fa"
 VTKBase = "4004b06d-e244-455f-a6ce-a5f9919cc534"
 
+# [weakdeps]
+# HDF5 = "f67ccb44-e63f-5c2f-98bd-6dc0ccc4ba2f"
+
+# [extensions]
+# WriteVTKHDFExt = "HDF5"
+
 [compat]
 CodecZlib = "0.7"
 FillArrays = "0.13, 1.0"
diff --git a/ext/WriteVTKHDFExt.jl b/ext/WriteVTKHDFExt.jl
new file mode 100644
index 00000000..3b280532
--- /dev/null
+++ b/ext/WriteVTKHDFExt.jl
@@ -0,0 +1,13 @@
+module WriteVTKHDFExt
+
+    using HDF5
+    import WriteVTK: vtk_grid, num_points
+    using WriteVTK: VTKHDF5, VTKHDFUnstructuredGrid, VTKUnstructuredGrid
+    using WriteVTK: UnstructuredCoord, CellVector
+    
+    __init__() = println("Pkg extension loaded!")
+    
+    
+
+
+end
\ No newline at end of file
diff --git a/src/WriteVTK.jl b/src/WriteVTK.jl
index a41ca6cf..70b9b034 100644
--- a/src/WriteVTK.jl
+++ b/src/WriteVTK.jl
@@ -13,6 +13,9 @@ export VTKPointData, VTKCellData, VTKFieldData
 export PolyData
 export pvtk_grid
 
+# vtkhdf needed
+export VTKHDF5, vtkhdf_open_timeseries, vtkhdf_append_timeseries_dataset
+
 import CodecZlib: ZlibCompressorStream
 import TranscodingStreams
 
@@ -163,6 +166,9 @@ include("gridtypes/unstructured/polydata.jl")
 # Parallel DataSet Formats
 include("gridtypes/pvtk_grid.jl")
 
+# vtkhdf formats
+include("gridtypes/vtk_hdf.jl")
+
 # Convenient high-level tools.
 include("utils/array.jl")
 include("utils/surface.jl")
diff --git a/src/gridtypes/vtk_hdf.jl b/src/gridtypes/vtk_hdf.jl
new file mode 100644
index 00000000..73ae2682
--- /dev/null
+++ b/src/gridtypes/vtk_hdf.jl
@@ -0,0 +1,262 @@
+using HDF5: HDF5
+
+abstract type VTKFileType end
+struct VTKHDF5 <: VTKFileType end
+struct VTKXML <: VTKFileType end
+
+struct VTKHDFUnstructuredGrid <: UnstructuredVTKDataset end
+
+struct VTKHDF5File <: VTKFile
+    h5file # an HDF5.File
+    version::AbstractString
+    grid_type::AbstractString
+    Npts::Int
+    Ncls::Int
+end
+
+# for unstructured
+struct VTKHDFTimeSeries{T<:AbstractFieldData}
+    vtkhdf::VTKHDF5File
+    name::AbstractString
+end
+
+# check if file is open
+Base.isopen(file::VTKHDF5File) = isopen(file.h5file)
+
+function Base.close(file::VTKHDF5File)
+    if isopen(file)
+        close(file.h5file)
+    end
+end
+
+function vtk_grid(
+    ::VTKHDF5,
+    filename,
+    points,
+    cells;
+    kws...)
+    vtk_grid(
+        VTKHDFUnstructuredGrid(),
+        filename, points, cells; kws...)
+end
+
+function vtk_grid(
+    dtype::VTKHDFUnstructuredGrid,
+    filename::AbstractString,
+    points::UnstructuredCoords,
+    cells::CellVector;
+    kwargs...)
+
+    Npts = num_points(dtype, points)
+    Ncls = length(cells)
+
+    # need types, offsets, connectivity for unstructured
+    # copied from unstructured.jl, but offset has a zero at the beginnining
+    # for VTKHDF case 
+
+    # vtk cell information
+    IntType = connectivity_type(cells)::Type{<:Integer}
+    # Create data arrays.
+    offsets = Array{IntType}(undef, Ncls + 1)
+    offsets[1] = 0
+
+    types = Array{UInt8}(undef, Ncls)
+
+    Nconn = 0     # length of the connectivity array
+    if Ncls >= 1  # it IS possible to have no cells
+        offsets[2] = length(cells[1].connectivity)
+    end
+
+    for (n, c) in enumerate(cells)
+        Npts_cell = length(c.connectivity)
+        Nconn += Npts_cell
+        types[n] = cell_type(c).vtk_id
+        if n >= 2
+            offsets[n+1] = offsets[n] + Npts_cell
+        end
+    end
+
+    # Create connectivity array.
+    conn = Array{IntType}(undef, Nconn)
+    n = 1
+    for c in cells, i in c.connectivity
+        # We transform to zero-based indexing, required by VTK.
+        conn[n] = i - one(i)
+        n += 1
+    end
+
+    h5file = h5open("$filename.vtkhdf", "w")
+
+    # write attributes
+    VTKHDF_group = create_group(h5file, "VTKHDF")
+    HDF5.attributes(VTKHDF_group)["Version"] = [2, 0]
+
+    type = "UnstructuredGrid"
+    h5_dspace = HDF5.dataspace(type)
+    h5_dtype = HDF5.datatype(type)
+    HDF5.h5t_set_cset(h5_dtype, HDF5.H5T_CSET_ASCII)
+    attr = create_attribute(VTKHDF_group, "Type", h5_dtype, h5_dspace)
+    write_attribute(attr, h5_dtype, type)
+
+    VTKHDF_group["NumberOfConnectivityIds"] = [Nconn]
+    VTKHDF_group["NumberOfPoints"] = [Npts]
+    VTKHDF_group["NumberOfCells"] = [Ncls]
+    VTKHDF_group["Points"] = points
+    VTKHDF_group["Types"] = types
+    VTKHDF_group["Connectivity"] = conn
+    VTKHDF_group["Offsets"] = offsets
+
+    VTKHDF5File(h5file, "2.0", type, Npts, Ncls)
+end
+
+"""
+Check for Steps group, maybe create it, and add to Steps.
+
+Return VTKHDFTimeSeries
+
+"""
+function vtkhdf_open_timeseries(
+    vtkhdf::VTKHDF5File,
+    name::AbstractString,
+    data_type::Union{VTKCellData,VTKPointData},
+    vec_dim=1
+)
+    root = vtkhdf.h5file["VTKHDF"]
+
+    if !haskey(root, "Steps")
+        steps = create_group(root, "Steps")
+        # initialize num_steps to zero
+        num_steps = 0
+        attrs(steps)["NSteps"] = num_steps
+
+        create_dataset(steps, "Values", Float64, dataspace((0,), (-1,)), chunk=(100,))
+
+        # offsets that are just all zeros
+        create_dataset(steps, "PartOffsets", Int64, dataspace((0,), (-1,)), chunk=(100,))
+        create_dataset(steps, "PointOffsets", Int64, dataspace((0,), (-1,)), chunk=(100,))
+        create_dataset(steps, "CellOffsets", Int64, dataspace((0,), (-1,)), chunk=(100,))
+        create_dataset(steps, "ConnectivityIdOffsets", Int64, dataspace((1, 0), (-1, -1)), chunk=(1, 100))
+        create_dataset(steps, "NumberOfParts", Int64, dataspace((0,), (-1,)), chunk=(100,))
+    else
+        steps = root["Steps"]
+    end
+
+    # check for and maybe create CellDataOffsets of PointDataOffsets respectively
+    if data_type isa VTKCellData
+        if vec_dim == 1
+            data_size = dataspace((0,), (-1,))
+            chunk_size = (vtkhdf.Ncls,)
+        else
+            data_size = dataspace((vec_dim, 0), (-1, -1))
+            chunk_size = (vec_dim, vtkhdf.Ncls)
+        end
+
+        # where data is stored
+        if !haskey(root, "CellData")
+            CellData = create_group(root, "CellData")
+        else
+            CellData = root["CellData"]
+        end
+        create_dataset(CellData, name, Float64, data_size, chunk=chunk_size)
+
+        if !haskey(steps, "CellDataOffsets")
+            CellDataOffsets = create_group(steps, "CellDataOffsets")
+        else
+            CellDataOffsets = steps["CellDataOffsets"]
+        end
+        # where offsets are stored
+        create_dataset(CellDataOffsets, name, Int64, dataspace((0,), (-1,)), chunk=(100,))
+    end
+
+    if data_type isa VTKPointData
+        if !haskey(root, "PointData")
+            CellData = create_group(root, "PointData")
+        else
+            CellData = root["PointData"]
+        end
+        # where data is stored
+        create_dataset(CellData, name, Float64, (0,))
+
+        if !haskey(steps, "PointDataOffsets")
+            PointDataOffsets = create_group(steps, "PointDataOffsets")
+        else
+            PointDataOffsets = steps["PointDataOffsets"]
+        end
+        create_dataset(PointDataOffsets, name, Int64, (0,))
+
+    end
+
+    VTKHDFTimeSeries{typeof(data_type)}(vtkhdf, name)
+end
+
+# check for vector case of size(N, M), instead  of size(N,)
+# also handle chunking?
+function append_and_resize(dset, array)
+    dset_size, dset_max_size = HDF5.get_extent_dims(dset)
+
+    array_dims = size(array)
+    data_size = array_dims[end]
+    prev_size = dset_size[end]
+
+    # handle vector case, e.g. (3, N)
+    if length(array_dims) == 2
+        new_size = (array_dims[1], data_size + prev_size)
+        HDF5.set_extent_dims(dset, new_size)
+        dset[:, 1+prev_size:end] = array
+    else
+        new_size = (data_size + prev_size,)
+        HDF5.set_extent_dims(dset, new_size)
+        dset[1+prev_size:end] = array
+    end
+end
+
+Base.setindex!(
+    series::VTKHDFTimeSeries{VTKCellData},
+    data,
+    time::Float64
+) = vtkhdf_append_timeseries_dataset(series, time, data)
+
+function vtkhdf_append_timeseries_dataset(
+    series::VTKHDFTimeSeries{VTKCellData},
+    time::Float64,
+    data
+)
+    root = series.vtkhdf.h5file["VTKHDF"]
+    steps = root["Steps"]
+
+    # append and resize the underlying datasets
+
+    # CellDataOffsets, append previous offset + length(CellData)
+    current_len = size(root["CellData"][series.name])[end]
+    append_and_resize(steps["CellDataOffsets"][series.name], [current_len])
+    # CellData, append 'data'
+    append_and_resize(root["CellData"][series.name], data)
+
+    # check the number of CellData datasets matches number of time steps
+    num_datasets = current_len ÷ size(data)[end]
+
+    if num_datasets == length(steps["Values"])
+        # increment NSteps
+        attrs(steps)["NSteps"] += 1
+
+        # Values, append time
+        append_and_resize(steps["Values"], [time])
+
+
+
+        # PartOffsets, append 0
+        append_and_resize(steps["PartOffsets"], [0])
+
+        # PointOffsets, append 0
+        append_and_resize(steps["PointOffsets"], [0])
+
+        # CellOffsets, append 0
+        append_and_resize(steps["CellOffsets"], [0])
+
+        # ConnectivityIdOffsets, append 0
+        append_and_resize(steps["ConnectivityIdOffsets"], [0][:, :])
+
+        # NumberOfParts, append 1
+        append_and_resize(steps["NumberOfParts"], [1])
+    end # need some sort of reasonable else condition to see if time steps are in sync or not
+end
\ No newline at end of file