diff --git a/NAMESPACE b/NAMESPACE
index b883ce1..18ec20a 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,10 +1,13 @@
 # Generated by roxygen2: do not edit by hand
 
 export(petGrid)
+export(speiGrid)
 importFrom(SPEI,hargreaves)
+importFrom(SPEI,spei)
 importFrom(SPEI,thornthwaite)
 importFrom(abind,abind)
 importFrom(magrittr,"%>%")
+importFrom(magrittr,extract2)
 importFrom(transformeR,array3Dto2Dmat)
 importFrom(transformeR,checkDim)
 importFrom(transformeR,getCoordinates)
diff --git a/R/petGrid.R b/R/petGrid.R
index ecda03e..712fd1d 100644
--- a/R/petGrid.R
+++ b/R/petGrid.R
@@ -92,7 +92,7 @@ petGrid <- function(tasmin = NULL,
     attr(pet.grid$Variable, "units") <- "mm.month-1"
     attr(pet.grid$Variable, "daily_agg_cellfun") <- "sum"
     attr(pet.grid$Variable, "monthly_agg_cellfun") <- "sum"
-    attr(pet.grid$Variable, "verification_time") <- "MM"
+    attr(pet.grid$Variable, "time_resolution") <- "MM"
     attr(pet.grid, "origin") <- paste0("Calculated with R package 'SPEI' v",
                                        packageVersion("SPEI"), "using R package 'drought4R' v",
                                        packageVersion("drought4R"))
diff --git a/R/speiGrid.R b/R/speiGrid.R
new file mode 100644
index 0000000..ff899ee
--- /dev/null
+++ b/R/speiGrid.R
@@ -0,0 +1,83 @@
+##     speiGrid.R Computation of Standardized Precipitation(-Evapotranspiration) Index Grids
+##
+##     Copyright (C) 2017 Santander Meteorology Group (http://www.meteo.unican.es)
+##
+##     This program is free software: you can redistribute it and/or modify
+##     it under the terms of the GNU General Public License as published by
+##     the Free Software Foundation, either version 3 of the License, or
+##     (at your option) any later version.
+## 
+##     This program is distributed in the hope that it will be useful,
+##     but WITHOUT ANY WARRANTY; without even the implied warranty of
+##     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+##     GNU General Public License for more details.
+## 
+##     You should have received a copy of the GNU General Public License
+##     along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+#' @title Computation of Standardized Precipitation(-Evapotranspiration) Index Grids
+#' @description Returns SPI/SPEI grid using precipitation (and PET for SPEI) input grids
+#' @param pr.grid Precipitation grid (monthly accumulated values, in mm)
+#' @param et0.grid Potential evapotranspiration grid (same units as \code{pr.grid})
+#' @param scale Integer. Time scale at which the SPEI/SPI are computed. Default to 3 (months)
+#' @param ... Further arguments passed to \code{\link[SPEI]{spei}}
+#' @details The function is a wrapper of function \code{\link[SPEI]{spei}} from package \pkg{SPEI} adapted
+#' to \strong{climate4R} input grids
+#' @return A \strong{climate4R} grid with SPEI/SPI data
+#' @importFrom magrittr %>% extract2
+#' @importFrom transformeR redim checkDim getSeason getTimeResolution getShape subsetGrid array3Dto2Dmat
+#' @importFrom SPEI spei
+#' @importFrom abind abind
+#' @export
+#' @seealso \code{\link{petGrid}}, for PET calculation
+#' @examples 
+#' # By default, et0.grid is null, and SPI is computed from precipitation: 
+#' data("pr.cru.iberia")
+#' spi3 <- speiGrid(pr.grid = pr.cru.iberia, scale = 3)
+#' ## If PET is used, then SPEI is calculated
+#' data("tas.cru.iberia")
+#' et0.grid <- petGrid(tas = tas.cru.iberia, method = "thornthwaite")
+#' spei3 <- speiGrid(pr.cru.iberia, et0.grid = et0.grid, scale = 3)
+
+speiGrid <- function(pr.grid, et0.grid = NULL, scale = 3, ...) {
+    pr.grid <- redim(pr.grid, member = TRUE)
+    if (!is.null(et0.grid)) {
+        et0.grid <- redim(et0.grid, member = TRUE)
+        checkDim(pr.grid, et0.grid)
+    }
+    if (!identical(1:12, getSeason(pr.grid))) stop("The input grid must encompass a whole-year season", call. = FALSE)
+    if (getTimeResolution(pr.grid) != "MM") stop("A monthly input grid is required for SPI/SPEI computation", call. = FALSE)
+    coords <- getCoordinates(pr.grid)
+    dimNames <- getDim(pr.grid)
+    n.mem <- getShape(pr.grid, "member")
+    method <- ifelse(is.null(et0.grid), "SPI", "SPEI")
+    message("[", Sys.time(), "] Computing ", method, " ...")
+    spei.list <- lapply(1:n.mem, function(x) {
+        pr <- subsetGrid(pr.grid, members = x, drop = TRUE) %>% extract2("Data") %>% array3Dto2Dmat()
+        pet <- if (is.null(et0.grid)) {
+            matrix(0, nrow = nrow(pr), ncol = ncol(pr))
+        } else {
+            subsetGrid(et0.grid, members = x, drop = TRUE) %>% extract2("Data") %>% array3Dto2Dmat()
+        }
+        wbalance <- pr - pet
+        pt <- pet <- NULL
+        index <- spei(wbalance, scale, na.rm = TRUE) %>% extract2("fitted") %>%  mat2Dto3Darray(x = coords$x, y = coords$y)
+    })
+    message("[", Sys.time(), "] Done")
+    ## Recover the grid structure -----------------------
+    pr.grid$Data <- do.call("abind", c(spei.list, along = -1L)) %>% unname()
+    attr(pr.grid$Data, "dimensions") <- dimNames
+    pr.grid <- redim(pr.grid, drop = TRUE)
+    pr.grid$Variable$varName <- paste(method, scale, sep = "_")
+    longname <- ifelse(method == "SPI", "Standardized Precipitation Index", "Standardized Precipitation-Evapotranspiration Index")
+    attr(pr.grid$Variable, "longname") <- paste(longname, scale)
+    attr(pr.grid$Variable, "units") <- "n/d"
+    attr(pr.grid$Variable, "daily_agg_cellfun") <- "sum"
+    attr(pr.grid$Variable, "monthly_agg_cellfun") <- "sum"
+    attr(pr.grid$Variable, "time_resolution") <- "MM"
+    attr(pr.grid, "origin") <- paste0("Calculated with R package 'SPEI' v",
+                                       packageVersion("SPEI"), "using R package 'drought4R' v",
+                                       packageVersion("drought4R"))
+    attr(pr.grid, "URL") <- "https://github.com/SantanderMetGroup/drought4R"
+    invisible(pr.grid)
+}
\ No newline at end of file
diff --git a/man/speiGrid.Rd b/man/speiGrid.Rd
new file mode 100644
index 0000000..72956d8
--- /dev/null
+++ b/man/speiGrid.Rd
@@ -0,0 +1,39 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/speiGrid.R
+\name{speiGrid}
+\alias{speiGrid}
+\title{Computation of Standardized Precipitation(-Evapotranspiration) Index Grids}
+\usage{
+speiGrid(pr.grid, et0.grid = NULL, scale = 3, ...)
+}
+\arguments{
+\item{pr.grid}{Precipitation grid (monthly accumulated values, in mm)}
+
+\item{et0.grid}{Potential evapotranspiration grid (same units as \code{pr.grid})}
+
+\item{scale}{Integer. Time scale at which the SPEI/SPI are computed. Default to 3 (months)}
+
+\item{...}{Further arguments passed to \code{\link[SPEI]{spei}}}
+}
+\value{
+A \strong{climate4R} grid with SPEI/SPI data
+}
+\description{
+Returns SPI/SPEI grid using precipitation (and PET for SPEI) input grids
+}
+\details{
+The function is a wrapper of function \code{\link[SPEI]{spei}} from package \pkg{SPEI} adapted
+to \strong{climate4R} input grids
+}
+\examples{
+# By default, et0.grid is null, and SPI is computed from precipitation: 
+data("pr.cru.iberia")
+spi3 <- speiGrid(pr.grid = pr.cru.iberia, scale = 3)
+## If PET is used, then SPEI is calculated
+data("tas.cru.iberia")
+et0.grid <- petGrid(tas = tas.cru.iberia, method = "thornthwaite")
+spei3 <- speiGrid(pr.cru.iberia, et0.grid = et0.grid, scale = 3)
+}
+\seealso{
+\code{\link{petGrid}}, for PET calculation
+}