diff --git a/include/base.h b/include/base.h
index d6e24e6..b24d844 100644
--- a/include/base.h
+++ b/include/base.h
@@ -67,6 +67,24 @@ struct arrow_t {
     uint z;
 };
 
+/*
+ * Structure describing a transition
+ */
+struct transition_t {
+    uint id;
+    struct condition_t *parent;
+    struct arrow_t *arrows;
+};
+
+/*
+ * Structure describing a condition
+ */
+struct condition_t {
+    uint id;
+    struct condition_t *parent;
+    struct arrow_t *arrows;
+};
+
 struct space_unit_t
 {
     bool lock;
@@ -78,9 +96,9 @@ struct space_t
 {
     // Dimensions of space.
     // Note that a value 0 is not allowed, minimum is 1
-    int x_dim;
-    int y_dim;
-    int z_dim;
+    uint x_dim;
+    uint y_dim;
+    uint z_dim;
 
     struct space_unit_t *units; // (flat) arraw of space_unit_t elements :
                                 // - lenght is x_dim * y_dim * z_dim
@@ -93,8 +111,8 @@ struct space_t
 struct state_t
 {
     // Metadata
-    int id;
-    int owner_id;
+    uint id;
+    uint owner_id;
     time_t date;
 
     struct space_t *space;
@@ -103,13 +121,13 @@ struct state_t
 struct model_t {
 
     // Metadata
-    int id;
-    int owner_id;
+    uint id;
+    uint owner_id;
     time_t date;
     union version
     {
-        int major;
-        int minor;
+        uint major;
+        uint minor;
     };
 
     // User friendly metadata
@@ -117,12 +135,12 @@ struct model_t {
     char *model_name;
 
     // Model parameters
-    int multiplicity;           // number of sites in a space_unit
-    int dimension;              // number of space dimensions
+    uint multiplicity;           // number of sites in a space_unit
+    uint dimension;              // number of space dimensions
 
     // Simulation parameters
-    int max_thread;
-    int max_cycles;
+    uint max_thread;
+    uint max_cycles;
 
     // Handler to the current space of the model
     struct space_t *space;
@@ -130,17 +148,25 @@ struct model_t {
     // Handler to the saved states of the model
     struct state_t **states;
 
+    // Array of conditions
+    struct condition_t *conditions;
+
+    // Array of transitions
+    struct transition_t *transitions;
 };
 
 struct worker_t
 {
-    int id;
-    int thread_num;
-    int status;
+    uint id;
+    uint status;
+    struct arrow_t *elected_arrow;
 };
 
 struct scheduler_t
 {
-    int id;
-    struct model_t **model;      // Queue (array) of waiting models
+    uint id;
+    uint n_workers;
+    bool pleaseStop;
+    struct model_t **models;      // Queue (array) of waiting models
+    struct worker_t **workers;   // Workers array
 };
diff --git a/src/scheduler.c b/src/scheduler.c
index d2a82f0..5ab624d 100644
--- a/src/scheduler.c
+++ b/src/scheduler.c
@@ -26,82 +26,115 @@
 
 /* -------------------------------------------------------------------------- */
 
+//
+//
+//
+//
 static int sched_new_id (void)
 {
     static int id = 0;
     return id;
 }
 
-
+//
+//
+//
+//
 static int sched_run(struct scheduler_t *self,
               struct parameters_t *parameters,
               struct model_t *model)
 {
     int n_threads = omp_get_max_threads();
     int n_arrows;
-    int n_workers;
     int workers_per_cycle;
     int max_cycles;
     int max_workers;
-    bool pleaseStop;
-    struct worker_t **workers;
 
-    //XXX
-    n_arrows = 10000000000;
+    self->pleaseStop = false;
+    self->n_workers = 0;
+    n_arrows = 2000;
     max_cycles = 1;
-    workers_per_cycle = (n_arrows / n_threads) + 1;
-    max_workers = workers_per_cycle * max_cycles;
+    workers_per_cycle = (int) round((double)n_arrows / (double)n_threads);
+    max_workers = workers_per_cycle * n_threads * max_cycles;
 
-    printlog("Need %d workers per cycle (%d arrows to work on) : %d\n",
+    printlog("Begin simulation of model %d with %d workers per cycle"
+             " (%d arrows to work on)\n",
+             model->id,
              workers_per_cycle,
              n_arrows);
 
-    pleaseStop = false;
-    n_workers = -1;
-
     #pragma omp parallel
     {
         int thread_num;
         thread_num = omp_get_thread_num();
 
-        while (!pleaseStop) {
-            workers[thread_num]->id = n_workers;
-            worker_start(workers[thread_num], self);
+        while (!self->pleaseStop) {
+            // Get an ID;
+            self->workers[thread_num]->id = self->n_workers;
+
+            // Elect an arrow (XXX TODO)
+            self->workers[thread_num]->elected_arrow = NULL;
+
+            // Launch evaluation
+            worker_start(self->workers[thread_num], self);
+
+            // Check if this is the end
+            if (self->n_workers > max_workers) {
+                self->pleaseStop = true;
+            }
         }
     }
 
-    printlog("End of simulation\n");
-
-    free(workers);
+    printlog("End of simulation (%d workers executed)\n",
+             self->n_workers);
 
     return SCHED_NORMAL_EXIT;
 }
 
+//
+//
+//
+//
 int sched_start (struct scheduler_t *self, struct parameters_t *parameters)
 {
     int n_threads = omp_get_max_threads();
-    int n_arrows;
-    int n_workers;
-    int workers_per_cycle;
-    int max_cycles;
-    int max_workers;
-    bool pleaseStop;
-    struct worker_t **workers;
-
+    int n_models = 20;
+    int returnValue;
     self->id = sched_new_id();
 
-    printlog("Hey, I'm the scheduler %d and I can work with %d threads !\n",
+    printlog("Scheduler initialized with %d threads\n",
              self->id,
              n_threads);
 
-    workers = calloc(n_threads, sizeof(struct worker_t*));
-    for (int i = 0; i < n_threads; i++) {
-        workers[thread_num] = calloc(1, sizeof(struct worker_t));
+    // Allocating models
+    self->models = calloc(n_models, sizeof(struct model_t*));
+    for (int i = 0; i < n_models; i++) {
+        self->models[i] = calloc(1, sizeof(struct model_t));
+        //XXX populate model
     }
 
-    sched_run(self, parameters, NULL);
-
+    // Allocating workers
+    self->workers = calloc(n_threads, sizeof(struct worker_t*));
     for (int i = 0; i < n_threads; i++) {
-        free(workers[thread_num]);
+        self->workers[i] = calloc(1, sizeof(struct worker_t));
     }
+
+    // Run
+    #pragma omp for
+    for (int i = 0; i < n_models; i++)
+        returnValue = sched_run(self, parameters, self->models[i]);
+
+    // Freeing workers
+    for (int i = 0; i < n_threads; i++) {
+        free(self->workers[i]);
+    }
+    free(self->workers);
+
+    // Freeing models
+    for (int i = 0; i < n_models; i++) {
+        free(self->models[i]);
+    }
+    free(self->models);
+
+    return returnValue;
 }
diff --git a/src/worker.c b/src/worker.c
index 77a1d7f..00985e2 100644
--- a/src/worker.c
+++ b/src/worker.c
@@ -35,20 +35,11 @@ static int worker_new_id (void)
 
 void worker_start(struct worker_t *self, struct scheduler_t *scheduler)
 {
-    unsigned int random_time;
-
-    random_time = (unsigned int)(rand() % 2);
-
-    printlog("Coucou, c'est le worker %d (et je vais dormir %d s)\n",
-            self->id,
-            random_time);
-
-    double a = (double)self->id * 42 / ((double)self->id + 2);
-    truc = a;
-    //sleep(random_time);
-
-    printlog("Fin du worker %d\n",
-         self->id);
-
+    // Locking ressources
+    #pragma omp critical
+    {
+        scheduler->n_workers++;
+        //XXX lock space units
+    }
     self->status = WORKER_NORMAL_EXIT;
 }